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. And that's what we get for m. And then if we want to solve for b, we literally can just substitute back into either equation, but this equation right here is simpler. And so if we wanted to solve for b there, we can solve for b in terms of m. We just subtract m times the mean of x's from both sides. We get b is equal to the mean of the y's minus m times the mean of the x's.