So they don't tell us how many subdivisions to make for our approximation, so that's up to us to decide. Let's say we went with three subdivisions. Let's say we wanted to make them equal. They don't have to be, but let's say we do. So the first one would go from two to four, the next one would go from four to six, and the next one would go from six to eight. If we do a left Riemann sum, you use the left side of each of these subdivisions in order to find the height. You evaluate the function at the left end of each of those subdivisions for the height of our approximating rectangles.