Implicit differentiation. And what I want you to keep in the back of your mind the entire time is that it's just an application of the chain rule. So let's apply the derivative operator to both sides of this. So it's the derivative with respect to x of x squared plus y squared, x squared plus y squared on the left-hand side of our equation, and then that's going to be equal to the derivative with respect to x on the right-hand side. I'm just doing the same exact thing to both sides of this equation. Now, if I take the derivative of the sum of two terms, that's the same thing as taking the sum of the derivative. So this is going to be the same thing as the derivative with respect to x of x squared plus the derivative with respect to x of y squared.