And we're going to approximate it by constructing four rectangles under the curve of equal width. So let's first think about what those rectangles look like. So four rectangles of equal width. So it would look like that, like that, and like that. And I haven't really defined the top of the rectangles just yet. But let's think about what those widths have to be if they're going to be equal width. And we can call that width delta x.