Exercise $4$

$($a$)$Find the Riemann sum for

$f(x)=3-2x,0x\frac{3}{2}$

with three approximating rectangles, taking the sample points at the left endpoints.

$($b$)$Obtain the exact value of ${\displaystyle \underset{0}{\overset{\frac{3}{2}}{}}}(3-2x)dx$ by interpreting it in terms of area $($do not evaluate the integral$).$