i am not sure what you exactly need. buti guess you want to show the answer after you clicked on the field once.
in your case when putting it in mouseClicked, you only do it for one frame, the next frame the original question is shown again.
so what you have to do is create a boolean variable showing the question if its false, and the answer if its true and then only change the state of that variable.


boolean showAnswer = false;


void setup() {
  size (480, 480);
  smooth();
}

void draw() {
  background(255);
  int[] numbers = new int[2];
  QuestionOne();
  if(showAnswer)answer();
}

void QuestionOne() {
  fill(255);
  rect(50, 50, 40, 20);
  fill(0);
  text("3 + 5 =", 15, 40);
  text("8", 60, 65);
 
}

void answer() {

    background(255);
   fill(0);
    text("8 + 6 =", 15, 40);
}

void mouseClicked(){
  if (mouseX > 50 && mouseX < 90 && mouseY > 50 && mouseY < 70)showAnswer = true;

}