So what's the area of this face? Well, we know that the area of a circle is equal to pi r squared. If we know the radius of this face, we can figure out the area of the face. Well, what's the radius? Well, the radius is just the height of that original rectangle, and for any x, the height over here is going to be equal to f of x, and in this case, f of x is x squared. So over here, our radius is equal to x squared. So the area of the face for a particular x is going to be equal to pi times f of x squared.