So I'm assuming you've paused the video and had a go at it, so let's think about it. So if we want to find the volume, one way to think about it is we could take, we could take the volume of, we could approximate the volume as the volume of these individual triangles. So that would be the area of each of these triangles times some very small depth, some very small depth, so let me just shade it in to show the depth, some very small depth, which we could call, we could call dx. So once again, we could find the volume of each of these by finding the area, the cross-sectional area there, and then multiplying that times a little bit, a little dx, a little dx, which would give us 3, so this is a little dx, which would give us 3-dimensional, that's our dx, I could write that a little bit neater, dx to give us a little bit of 3-dimensional depth. So how could we, what is the volume of one of these figures going to look like? Well, if we say, let me just call this height h, and we know that h is going to be f of x minus g of x, that's this distance right over here, so let's call that h. We know that h is going to be, and maybe I should say h of x, because it is going to be a function of x, h of x is going to be f of x, f of x minus g of x, minus g of x. But given an h, what is going to be the area of this triangle?