Dez is right. Even if it's simple for you, you know it's 0 without any hesitation, some teachers like that you show your work step by step.

Like, rewrite the problem (as Dez said)
1+(-1)
Cancel out the + and - to make a minus.
Then subtract the one from the other one and specifically put " = 0"

I know, it seems silly, but the teacher wants to see your work. To see that you've actually put an effort towards solving the problem and didn't just use a calculator. Though, in more complex problems, where calculators are necessary, you still need to put the two numbers together and solve for what's being asked.