B is the answer because you want a number that will get you to zero. If you have -7, you want to get -7 to zero so in order to do that, you add seven. If you have 6, and you want to get it to 0, then you subtract 6. You have to find a number that will get you to 0 (sorry if none of this made sense, basically X has to equal the opposite of whats in the parathesis. ex. you have 6, X would be -6. If you have -7. X would be 7)
