F(x) = (ax b)/(cx-a). Show algebraically that f(f(x)) = x, irrespective of the values of a, b and c.

I've been fiddling around with this one for a while, and am kind of stumped. I would really appreciate it if someone could give me a bit of a pointer as to how to go about this question. Being honest, I should tell you that it is for an ignment. Because I don't want to steal credit for someone else's work, some general instruction, or links to helpful websites would probably be preferable to a worked solution. :D Thanks for any help in anticipation!