Change in y, so the difference between that y and that y is that right over there, over the change in x's. The change in that x, that x minus that x is exactly this over here. Now to simplify it, we can multiply both the numerator and the denominator by the mean in x. The mean of the x's. And I do that just so we don't have this in the denominator both places. And so if we multiply the numerator by the mean of the x's, we get the mean of the x's times the mean of the y's minus the mean of the x y's. All of that over, mean of the x's times the mean of the x's is just going to be the mean of the x's squared minus over here you have the mean of the x squared.