$[1/2$ Points$]$ SCALC$915\mathrm{.}1\mathrm{.}001.$MI$.$

Estimate the volume of the solid that lies below the surface $z=xy$ and above the following rectangle.

$R=\{(x,y)|4x10,0y4\}$

$($a$)$ 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.