So first, just simply solve both and then it become obvious.
The expression 3-(2+5), you can evaluate what's inside the parentheses first and then your answer will be in the form of a integer:
3-(2+5) = 3-7 = -4
With the second one, your answer will be in the form of a simplified expression because we don't know what x is:
3x-(2x+5) = 3x-2x-5 = x - 5
Also, in the second one, you have to expand the parentheses in order to simplify it.
