The expected value of X times Y, it can be approximated by the sample mean of the products of X and Y. This is going to be the sample mean of X and Y. You take each of your XY associations, take their product, and then take the mean of all of them. So that's going to be the product of X and Y, and then this thing right over here, the expected value of Y, that can be approximated by the sample mean of Y, and the expected value of X can be approximated by the sample mean of X. So what can the covariance of two random variables be approximated by? What can they be approximated by? Well, this right here is the mean of their product from your sample, minus the mean of your sample Ys times the mean of your sample Xs.