And we get this expression over here which we've been simplifying over the last couple of videos. We're going to try to simplify the expression as much as possible and then we're going to try to optimize, we're going to try to minimize this expression or find the m and b values that minimize it or that's, I guess you can call it, the best fitting line. Now to do that, it looks like we were just getting, making the algebra even hairier and hairier but this next step is going to simplify things a good bit. So just to show you that, what is, if I want to take the mean of all of the squared values of the y's. So that would be this, that would be y1 squared plus y2 squared plus all the way to yn squared. So I've summed n values, n squared values and then I want to divide it by n, since there are n values here, and this is the mean of the y's squared. That's how we can denote it just like that.