Hi. @Rissa_Ja There are a couple things wrong with the code but overall you're pretty much there. Below is a working example of the code with revision and some quick comments detailing them.

 #pragma strict
 
 var x : int = 2;
 
 var y : int = 3;
 
 //The total sum of x+y which will be compared against the answer.
 var answer : int = x+y;
 
 var z : int = 4;
 
 var HOB : int = 25;
 
 //The string which the player will type and will be compared.
 var sum : String = null;
 
 //Boolean to chech whether or not the question should be displayed.
 var shouldDisplayQuestion : boolean = false;
 
 function OnTriggerEnter (other : Collider)
 {
     //Player has entered collider and we want to display 
     //the question so we turn our boolean to true.
     shouldDisplayQuestion = true;
 }
 
 //Function which handles GUI output.
 function OnGUI ()
 {
     //An if statement to check for whether or not we should display the question.
     if(shouldDisplayQuestion)
     {
         //If true we then run the display question method.
         DisplayQuestion();
     }
 }
 
 //Function which handles the question.
 function DisplayQuestion ()
 {
     GUI.Label(Rect((Screen.width)/2-230, 130, 200, 50), "What is the sum of x+y?");
     //The string we wish to input with our text field.
     sum = GUI.TextField(Rect((Screen.width)/2-20,130,200,HOB), sum, 45);
     
     //Check for whether the button was pressed.
     if(GUI.Button(Rect((Screen.width)/2-230,420,200,HOB),"OK!"))
      {
          //If statement which compares the players string answer 
          //with that of x+y's answer parsed into a string.
          if(sum != answer.ToString())
          {
              Debug.Log("wrong answer, please try again");
          }
          else
          { 
              Destroy(gameObject);
              CoinSystem.coinscollect +=1;
              //We turn our boolean to false as the question as has been answered 
              //and we no longer want to show the menu.
              shouldDisplayQuestion = false;
          }
      }
 }

Now for a bit more information on the code the main issues with your code are:

1. Your GUI statements must be declared within or from the OnGUI function otherwise they will not be drawn and the console will throw an error.

2. The text field takes a string value and as such can only compare the x+y question as a string value so you have to parse your int's to a string to compare them.

3. To deal with the OnGUI function i've used a boolean to decide whether or not the OnGUI function should display the question based on whether the player has entered the trigger.

4. The text field requires a string value to change in this case i have used the sum variable.

That's pretty much it, if you need any more help or this code is causing problems don't hesitate to ask. I hope this helps! :)