There's no b here, there's no b over here, so the partial derivatives of either of these with respect to b is 0. Then over here, you have a negative 2n times the mean of y's as a coefficient on a b. So the partial derivative with respect to b is going to be minus 2n, or negative 2n times the mean of the y's. And then there's no b over here, and we do have a b over here, so it's plus 2mn times the mean of the x's. Plus 2mn times the mean of the x's. This is essentially the coefficient on the b over here. It was written in a mixed order, but all of these are constants from the point of view of b.