Answer: X and Y are variables which means they represent some number that you don't know. When you are working with Graphs, X and Y could be coordinates. When it wants you to solve for x you have to do the inverse opperation until you isolate the variable you are trying to solve. Basically, inverse opperations are if it is -30 on one side you would add 30 to both sides to cancel it out. When you isolate the variable it is only that one letter with everything else on the other side.
Explanation:
If this is confusing, let me know an example question and I can help you in the comments.