The rate of change of y with respect to x is the slope of our line. So can we use, and this is really all that we know, we could keep, actually we could go even further. We could take the second derivative here. The second derivative of y with respect to x, well that's going to be zero. The second derivative of a linear function, well it's going to be zero, we see that here. So this is all of the information that we have. We get this from the previous parts of the problem and we get this just taking the first and second derivatives of y equals mx plus b.