1. The problem statement, all variables and given/known data I'm working on a problem in my MAT-095: Algebraic Concepts class. The overall problem is to prove the Midpoint Formula for finding the midpoint between two points on a line. But in the process of working through the proof I ran into something that I don't really remember very well from the more basic classes... I thought it was true, so I decided to try and prove it to myself. And I came up with something that "proves" what I thought I knew, but I suck at proofs so I wanted to get a little "sanity check" if I could. 2. Relevant equations ( x - (x + y) ) = -y I said this was trivial. Heh. 3. The attempt at a solution subtract x from both sides, yielding -(x + y) = (-y -x) distribute out a -1 from the right hand side yielding -(x + y) = -1(y + x ) fill in the assumed 1 on the left hand side yielding -1(x + y) = -1( y + x) divide both sides by -1 yielding (x + y) = (y + x), which is true by the commutative property of addition. So, am I right, or did I miss something? Feel free to make fun of my ignorance, my math "skills" are pretty pathetic.