For the function given below, find a formula for the Riemann sum obtained by dividing the interval $a,b$ into $n$ equal subintervals and using the right$-$hand endpoint for each $c_k.$ Then take a limit of this sum as $n$ to calculate the area under the curve over $a,b.$

$f(x)=7x^3$ over the interval $0,1$

Find a formula for the Riemann sum.

$S_n=$