1. The problem statement, all variables and given/known data Trying to run through every problem i can in my book in preparation for my exam. I've solved this one before, but it slipped my mind how to do it : http://gyazo.com/9fcf9f3cef522c3d5eb1fa7d4ad04394 2. Relevant equations Working in a group, so group axioms I suppose. 3. The attempt at a solution I forgot where to start this one off, I was thinking : e = aa-1 a-1e = a-1aa-1 That wont get me anywhere though, any pointers would be appreciated.