You could think about it in terms of point slope, where you could say y minus some y that sits on that line is equal to the slope times x minus the corresponding x one. So x one comma y one sits on that line someplace. Actually, I like to write this point slope form like this sometimes. Y minus y one over x minus x one is equal to b. Because this comes straight out of the idea of, look, if x one and y one are on the line, the slope between any other point on the line and that point is going to be your slope of your line. So we could think about it any of these ways. So let's first find the slope of the tangent line, and that's where the derivative is useful.