So let's say that this is some arbitrary x right over here. This would be the point x, f of x. And let's say that this is, let's call this x plus h. So this distance right over here is going to be h. This right over here is going to be the point x plus h, f of x plus h. Now the whole underlying idea of the formal definition of limits is to find the slope of the secant line between these two points, and then take the limit as h approaches 0. As h gets closer and closer, this blue point is going to get closer and closer and closer to x, and this point is going to approach it on the curve, and the secant line is going to become a better and better and better approximation of the tangent line at x. So let's actually do that. So what's the slope of the secant line? Well, it's the change in your vertical axis, which is going to be f of x plus h minus f of x over the change in your horizontal axis.