a) Estimate the volume of the solid that lies below the surface z = xy and above the rectangle
R = {(x, y) | 0 < x < 6, 0 < y < 4}
Use a Riemann sum with m = 3, n = 2, and take the sample point to be the upper right corner of each square.
(b) Use the Midpoint Rule to estimate the volume of the solid in part (a).