So you have f of x plus h, g of x plus h, minus f of x, g of x, all of that over h, limit as h approaches zero. Now why did I put this big, awkward space here? Because just the way I've written it right now, it doesn't seem easy to algebraically manipulate. I don't know how to evaluate this limit. There doesn't seem to be anything obvious to do. And what I'm about to show you, I guess you could view it as a little bit of a trick. I can't claim that I would have figured it out on my own, maybe eventually if I were spending hours on it.