So let's say my function looks like this. And we care about the point x equals C, which is right over here. So that's the point x equals C. And then this value, of course, is going to be f of C. And one way that we can find the derivative at x equals C, or the slope of the tangent line at x equals C, is we could start with some other point, say some arbitrary x out here. So let's say this is some arbitrary x out here. So then this point right over there, this value, this y value, would be f of x. This graph, of course, is a graph of y equals f of x. And we can think about finding the slope of this line, this secant line between these two points.