The question mark should be 2, because you are trying to get x by itself.
So, because it is a negative, you need to add it so that it would be neutral.
3x - 2 = 7
Add 2 to both sides.
3x = 7 + 2
3x = 9
Divide by 3 on both sides,
x = 9/3
x = 3
So, your answer is add 2.
~Hope I helped!~