That's the area of this face right over here. What is the volume of our disk? We're just going to have to multiply that times our depth. So it's going to be that times dx. And we want to take the sum of all of these disks from a to b. And we're going to take the sum of them and then take the limit as the dx's get smaller and smaller and smaller, and we have an infinite number of these disks. And that means we're just going to take a definite integral.