If we want the volume of this thing, we just multiply it times the depth of each of those washers. So the volume of each of these washers are going to be pi times, pi times f of x, f of x squared minus g of x squared, the outer function squared over our interval minus our inner function squared over the interval, and then times our depth. Times our depth. That'd be the volume of each of these washers. Now, for each, and that's going to be defined at a given x in our interval, but for each x at this interval, we can define a new washer. So there could be a washer out here and a washer out here. So we're going to take the sum of all of those washers and take the limit as we have smaller and smaller depths and we have an infinite number of infinitely thin washers.