For every real number, is there a number that you can add to it to get zero? For example, say the real number is 2. Add -2, and what do you get?
The question means: if you are given a real number can you always find a real number, which when added to the one you were given gives an answer of 0. Given 5, use -5, because 5+(-5) 0. Given -5, use 5, because -5 + 5 0. You can always do this. The only questionable case is 0 itself. But: Given 0, use 0, because 0+0 0. This last example says the additive inverse of 0 is 0 itself.
