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I am creating a code that asks the user to choose a variable to solve for, it generates a question asking for that user-specified variable, and then the user can enter a number and check if they are correct. My code generates the question correctly, but it when the user submits their answer, I receive this message: HTTP Status 500 - Internal Server Error. I have included my jsp file, my calculation class, and my controller servlet.

               <!-- Heading for practicing the acceleration equation -->
                <h1>Practice the Equation:</h1>
                <p>Generate some questions for extra practice!</p>

                <!-- Choose a variable section -->
                <p>What variable do you want to solve for?</p>
                <!-- Drop down for selecting a variable -->
                <form action="displayQuestion" method="POST">
                    <select name = "variable" id="variable">
                        <option value="A">Acceleration</option>
                        <option value="V">Final Velocity</option>
                        <option value="U">Initial Velocity</option>
                        <option value="T">Time</option>
                    </select>
                    <input type="submit" value="Generate Question">
                </form>
                <!-- Placeholder for displaying the question -->
                <p>${question}</p>

                <!-- Answer input section -->
                <form action="practiceProblems" method="POST">
                    <!-- Text input field for entering the answer -->
                    <input type="text" id="answer" name="answer" placeholder="Enter answer...">
                    <!-- Button for checking the answer -->
                    <input type="submit" value="Answer">
                </form>
                <!-- Placeholder for displaying the result -->
                    <p>${result}</p>       

  // Method to generate a random question
  public String generateProblem(String variable)
  {
    String question = "";

    switch (variable)
    {
      case "A":
        question = "What is the acceleration when the initial velocity is 10 m/s, the final velocity is 30 m/s, and the time is 5 seconds?";
        break;
      case "V":
        question = "What is the final velocity when the acceleration is 5 m/s^2, the initial velocity is 0 m/s, and the time is 10 seconds?";
        break;
      case "U":
        question = "What is the initial velocity when the acceleration is 2 m/s^2, the final velocity is 20 m/s, and the time is 8 seconds?";
        break;
      case "T":
        question = "What is the time when the acceleration is 3 m/s^2, the initial velocity is 10 m/s, and the final velocity is 40 m/s?";
        break;
    }

    return question;
  }

  // Method to check the answer
  public String checkAnswer(String variable, String answer)
  {
    String result = "";

    switch (variable)
    {
      case "A":
        if (answer.equals("4"))
        {
          result = "Correct! The acceleration is 4 m/s^2.";
        }
        else if (!isValidNumber(answer))
        {
          result = "You entered an invalid input. Please enter a number or a decimal.";
        }
        else
        {
          result = "Incorrect. The acceleration is 4 m/s^2.";
        }
        break;
      case "V":
        if (answer.equals("40"))
        {
          result = "Correct! The final velocity is 40 m/s.";
        }
        else if (!isValidNumber(answer))
        {
          result = "You entered an invalid input. Please enter a number or a decimal.";
        }
        else
        {
          result = "Incorrect. The final velocity is 40 m/s.";
        }
        break;
      case "U":
        if (answer.equals("0"))
        {
          result = "Correct! The initial velocity is 0 m/s.";
        }
        else if (!isValidNumber(answer))
        {
          result = "You entered an invalid input. Please enter a number or a decimal.";
        }
        else
        {
          result = "Incorrect. The initial velocity is 0 m/s.";
        }
        break;
      case "T":
        if (answer.equals("10"))
        {
          result = "Correct! The time is 10 seconds.";
        }
        else if (!isValidNumber(answer))
        {
          result = "You entered an invalid input. Please enter a number or a decimal.";
        }
        else
        {
          result = "Incorrect. The time is 10 seconds.";
        }
        break;
    }

    return result;
  }

  // Method to check if a value is a valid number
  private static boolean isValidNumber(String value)
  {
    try
    {
      Double.parseDouble(value);
      return true;
    }
    catch (NumberFormatException e)
    {
      return false;
    }
  }

case "/practiceProblems":
        variable = request.getParameter("variable");
        answer = request.getParameter("answer");

        String result = solver.checkAnswer(variable, answer);

        // Set the generated question as an attribute to be used in JSP
        session.setAttribute("result", result);

        myURL = "practiceProblems.jsp";

        // forward the HTTP request to practiceProblems.jsp
        try
        {
          // send data to myURL
          request.getRequestDispatcher(myURL).forward(request, response);
        }
        catch (ServletException | IOException ex)
        {
          //go to the error page
          myErrorURL = "/WEB-INF/error.jsp";
          request.getRequestDispatcher(myErrorURL).forward(request, response);
        }

        break;

      case "/displayQuestion":
        variable = request.getParameter("variable");

        String question = solver.generateProblem(variable);
        session.setAttribute("question", question);

        myURL = "practiceProblems.jsp";

        // forward the HTTP request to practiceProblems.jsp
        try
        {
          // send data to myURL
          request.getRequestDispatcher(myURL).forward(request, response);
        }
        catch (ServletException | IOException ex)
        {
          //go to the error page
          myErrorURL = "/WEB-INF/error.jsp";
          request.getRequestDispatcher(myErrorURL).forward(request, response);
        }

        break;