Well, let me rewrite it using these integrals right up here. This is going to be equal to the limit as delta x approaches 0 of what's f of x plus delta x? Well, put x in right over here. You're going to get the definite integral from a to x plus delta x of f of t dt. And then from that, you are going to subtract this business, f of x, which we've already written as the definite integral from a to x of f of t dt. And then all of that is over delta x. Now, what does this represent?