I don't want you to be confused. You're saying that you're approximating the population's regression line from a sample of it. Now, this right here, so everything we've learned right now, this right here is the covariance, or this is an estimate of the covariance of X and Y. Now, what is this over here? I just said you could rewrite this very easily. This bottom part right here, you could write as the mean of X times X, that's the same thing as X squared, minus the mean of X times the mean of X. That's what the mean of X squared is.