In that case, you should just think any number that when you multiply it to each other, it must result must be -100. And when you add it to each other it will be "0" zero. So the answer to your question is 10. But when when you multiply it to each other, it must result to -100.. so the other one must be a positive 10 (x + 10) and the other one is (x - 10).
