I need to find the marginal profit as tied into this equation

R=2x(900+32x-x^2)

with Marginal Revenue =

a.) Additional revenue for x=14 to x=15

b.) Marginal revenue when X=14

c.) Compare b and a

The problem I'm having is that my marginal revenue looks nothing like my additional revenue hahaha

Marginal revenue is supposed to be gotten from the derivative of the equation that I wrote out as . Part of this is from me dropping the 900 from the inside the () because it didn't have a x factor. Am I correct in doing this? This is supposed to create a "per unit" sort of equation. Am I doing something wrong? Thanks in advance