Question $8$: Consider the curve $y=x^{3}x.$

a$)$ Approximate the area under the curve between $x=0$ and $x=4$ by a Riemann sum of four rectangles using right endpoints. Show your work. $(5$ pts$)$

b$)$ Find the exact area under the curve between $x=0$ and $x=4.(5$ pts$).$ Show your work.