Two numbers are called *additive inverses* if, when you add
them, you get 0. For example, 3 and −3 are additive inverses.
Every number has exactly one additive inverse. Your teacher may call
additive inverses “opposites”; they mean the same
thing.

To find the additive inverse of a number, just put a negative sign
in front of it, unless it has one already. If it does, then take it
away. A special case is 0; the additive inverse of 0 is just 0 again.
That's right — 0 is *its own* additive inverse. This
makes sense, though, since 0 + 0 = 0.