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. They're the coefficient in front of the b. The partial derivative of that with respect to b is just going to be the coefficient. And then finally, the partial derivative of this with respect to b is going to be 2nb, or 2nb to the first.