Please help and show work using limit of x>infinity method. I am primarily struggling with the part where algebra needs to be use to simplify the sigma sum notation into a infinity limit form

For the function given below, find a formula for the Riemann sum obtained by dividing the interval [0,21] into n equal subintervals and using the right-hand endpoint for each CK. Then take a limit of t n - co to calculate the area under the curve over [0,21] f(X) = 441 -x2 Write a formula for 'a Riemann sum for the function f(x) = 441 - x over the interval [0,21]