Or, one way to think about it is that this guy and that guy will cancel out. You could have also picked that guy and that guy. But what do we have left? We have the covariance of these two random variables, X and Y, are equal to the expected value of, I'll switch back to my colors just because this is the final result, the expected value of X times the expected value of the product of XY, the expected value of the product, the expected value of the product, minus, what is this? The expected value of Y, the expected value of Y times the expected value of X, times the expected value of X. Now, you can calculate these expected values if you know everything about the probability distribution or density functions for each of these random variables, or if you had the entire population that you're sampling from whenever you take an instantiation of these random variables. But let's say you just had a sample of these random variables.