Let's say that that is, well, I'll just write, that's our function f. That's our function f. And so you could have the point when x is equal to a. This is our x-axis. When x is equal to a, this is the point a f of a. You notice a f of a. And then we could take the slope between that and some arbitrary point. Let's call that x. So this is the point x f of x.