And that slope of the secant line between these two is going to approximate the slope of the tangent line at x. And so this right over here, this we would say is equal to f prime of x. This is a function of x. You give me an x, you give me an arbitrary x where the derivative is defined. I'm going to plug it into this. Whatever this ends up being, it might be some nice clean algebraic expression. And then I'm going to give you a number.