What is the area of the face of one of those disks? Well, it's going to be pi times the radius squared. In this case, the radius is going to be equal to the value of this inner function, which is just x. So pi times, and so this is just y is equal to x. And then we're going to multiply it times the depth, times the depth of each of these disks. And each of these disks are going to have a depth of dx. If you imagine a quarter that has an infinitely thin depth right over here, so it's going to be dx.