Either way, x0 and a are the same thing. So it's f of a times our delta x, times our width, our height times our width. So times delta, actually, let me write f of x0. I wanted to write f of x0 times delta x. What is our height of rectangle 2? It's f of x1 times delta x. f of x1 times delta x. What's our area of rectangle 3?