How exactly do i do b + c???
For b: you want to divide the area under the curve into rectangles.
For the expression on the left, you want your rectangles to be below the curve, with a bit of a curvy triangle under the curve not included.
For the expression on the right, you want your rectangles to be above the curve, with a bit of a curvy triangle above the curve included.
Maybe this image will help.
The green is for the expression on the left, and the yellow for the expression on the right. This is not the curve you're looking for though, it's just to illustrate what needs to be done.
It is intuitively obvious that having the rectangles go over the cruve will give a greater area than the area under the curve, and having the rectangles below the curve gives a smaller area than the one under the curve.