$(a)$ Using sigma notation for your sums, derive the formula for the right Riemann sum $R_{n}of$

the function $f(x)=(2-x{)}^{3}on$ the interval $2,4,$ using $n$ subintervals $of$ equal length.

$(b)$ Then, calculate the definite integral ${\displaystyle \underset{2}{\overset{4}{}}}f(x)dxas$ a $limitof$ right Riemann sums.