That is our yellow function right over there. Now, how do we figure out the area of the surface of one of these rings or one of these washers? Well, the area, let me do this in orange because I drew that ring in orange. So the area of the surface right over here in orange as a function of y is going to be equal to the area of the circle if I just consider the outer radius. And then I subtract out the area of the circle constructed by the inner radius. Just kind of subtract it out. So the outer circle radius, so it's going to be pi times outer radius squared minus pi times inner radius squared.