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. And we want this all to be a function of y. So the outer radius as a function of y is going to be what? Well, it might be easier to visualize.