I think you can appreciate this is simplifying the algebraic expression a good bit. This term right over here is going to be minus 2b, minus 2bn, times the mean of the y values. And then we have plus, we have this term right here, plus m squared times n times the mean of the x squared values. And then we have, almost there, home stretch, we have this over here which is plus 2mb times n times the mean of the x values. And then finally we have plus nb squared. So really in the last two, three videos, all we've done is we've simplified the expression for the sum of the squared differences from those end points to this line y equals mx plus b, right over here. So that is kind of, we're finished the hardcore algebra stage.