This error right here is going to be yn minus mxn plus b. Now, what we want to do, so if we wanted to just take the straight up sum of the errors, we could just sum these things up. But what we want to do is minimize the square of the error between each of these points, each of these end points in the line. So let me define, I'll do this in a new color. Let me define the squared error against this line as being equal to the sum of these squared errors. So this error right here, or error 1 we could call it, is y1 minus mx1 plus b. And we're going to square it.