You evaluate the function at the left end of each of those subdivisions for the height of our approximating rectangles. So we would use g of two to approximate for, or to set the height of our first approximating rectangle, just like that, and then we would use g of four for the next rectangle. So we would be right over there, and then you'd use g of six to represent the height of our third and our final rectangle right over there. Now, when it's drawn out like this, it's pretty clear that our left Riemann sum is going to be an overestimation. Why do we know that? Because these rectangles, the area that they're trying to approximate are always contained in the rectangles, and these rectangles have this surplus area, so they're always going to be larger than the areas that they're trying to approximate. And in general, if you have a function that's decreasing over the interval that we care about right over here, and strictly decreasing the entire time, if you use the left edge of each subdivision to approximate, you're going to have an overestimate, because the left edge, the value of the function there, is going to be higher than the value of the function at any other point in the subdivision.