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. You could even say 2bn, or 2nb is the partial derivative of that with respect to b. And we want to set this equal to 0. So it looks very complicated, but remember we're just trying to solve for the m's and the b's.