Well, we could view that as the limit as x approaches a of f of x minus f of a over x minus a. So this is literally just the slope between two points. So if you have your function f of x like this, this is the point a, f of a right over here. This right over here is the point x, f of x. This expression right over here is the slope between these two points. The change in our y value is f of x minus f of a. The change in our x value is x minus a.