1. The problem statement, all variables and given/known data For which numbers a, b, c, and d will the function f(x) = (ax + b) / (cx + d) satisfy f(f(x)) = x for all x (for which this equation makes sense)? 2. Relevant equations 3. The attempt at a solution I'm not sure what to do, which is my problem. So far I think that I can plug in (ax + b) / (cx + d) for x, because that is when it will satisfy f(f(x)) = x, right? So then I have: http://img26.imageshack.us/img26/6364/mathnumber8.jpg [Broken] So I'm not sure where I want to end up, or where to go from here :( Can someone help me? This is due tomorrow. All help is appreciated.