a$)(8$ pts$)$ Using sigma notation, write the midpoint Riemann sum to approximate the area bounded by the graph of $f(x)=x^2-1$ and the x $-$axis on the interval $[2,7],n=40,$ using the steps shown in class. $($You will calculate the sum in part $($b$).)$ Write the sum in the form: ${\displaystyle \underset{k=1}{\overset{n}{}}}f(\frac{?}{\text{b}\text{a}\text{r}}(x_k))x$ and then go to part b$).$