This is how we figured out the slope of our regression line. Or maybe a better way to think about it, if we assume in our regression line that the points that we have were a sample from an entire universe of possible points, then you could say that we are approximating the slope of our regression line. You might see this little hat notation in a lot of books. 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?