Question

What should be added to 8 to make the sum of 0

Answer 1

Conceptually, you are asking for the additive inverse of 8. By definition, in fact, the additive inverse of a number $x$ is a number $y$ such that $x+y=0$

You obtain the additive inverse of a number by switching its sign, so the additive inverse of 8 is -8. In fact, you have

$8+(-8) = 8-8=0$