Save correct value of a question in a variable.

I'm not sure exactly what you're asking. 

If you simply want to tell them the correct answer, then edit the feedback on the Incorrect layer.

If you want to replace what the user entered with the correct value, do this: Add a trigger that adjusts that slide's text-entry variable to whatever the correct answer is. Run the trigger when the timeline of the Incorrect layer starts. When the variable is adjusted, the updated value will show in the text-entry field. 

• IMHO, I think it's better to tell the user what the correct answer is via the feedback on the Incorrect layer. That lets them compare their answer with the correct answer. 

Here's more info about data-entry: Storyline 360: Working with Data-Entry Fields | Articulate - Community