So this length right over here is going to be the absolute value of f of x minus g of x. That's the length of the base. But if we want the area of that cross section, you can imagine, let me draw it at a little bit of an angle here. So you have a cross section of a square there. You have a cross section of a square here. And so the area of each of those cross sections is going to be the length of the base squared. And then you multiply that times the little dx's.