finding the area under a curve

In this task, you will practice finding the area under a non-linear function by using rectangles. You will use graphing skills in addition to the knowledge gathered in this unit. Sketch the graph of the function Y= 20X - X² and approximate the area under the curve in the interval [0,20] by dividing the area into the given numbers of rectangles

please please awnser parts A, B, and C
all posted in the photo attached!!​