Which property justifies the statement below? a+(-a)=0

Atheater wants to build movable steps that they can use to go on and off the stage. they want the steps to have enough space inside so they can also be used to store props. how much space is inside the steps?

Precalculus question, image attached.

Reinel saves 30% of every pay check. his paycheck last week was 560$. how much did reniel save last week?