(a) Estimate the volume of the solid that lies below the surface $ z = 1 + x^2 + 3y $ and above the rectangle $ R = [1, 2] \times [0, 3] $. Use a Riemann sum with $ m = n = 2 $ and choose the sample points to be lower left corners.
(b) Use the Midpoint Rule to estimate the volume in part (a).