1. Marginal Revenue

Hi!

I've been stuck on this question for the last hour... I just can't seem to figure it out.

The problem consists of 3 parts: A, B, C.
I've managed to figure out part A, but not B and C.

2. this is the marginal revenue function, the derivative of the total revenue function $\displaystyle MR(q)=810\, \left( 314896+3\,q \right) ^{- 0.5}- 1215\,q \left( 314896+3\, q \right) ^{- 1.5}$