This literally, this comes out of them being each other's inverses. We could have also said h of f of x will also be equal to x. Remember, f is going to map, or h is going to map from some x to h of x, and then f is going to map back to that original x. That's what inverses do. So that's because they are inverses. This is by definition, this is what inverses do to each other. But then if you took the derivative of both sides of this, what would you get?