6. The profit selling x units of pipes is P(x)=420x-(1/4)x^2

a.Find the marginal profit function Pm
b.What is the marginal profit for 800 units?
c.What is the marginal profit for 900 units?
d.Determine the value of x that will provide the maximum profit

My Questions are:

1.In a. should I get the first derivative or the anti-derivative to have the marginal profit function?
2.In b & c. after getting the marginal profit function, should I replace x with 800 and 900 units?
3.In d. should I use trial and error in determining the value of x, or there is a easier way to do that?

* Please give me tips on how to solve this problem

*Just click the link for a clearer view of the question: ( Just look at question no.6 )

https://lh3.googleusercontent.com/-Lh968KtAvZU/UEs4hj_ZomI/AAAAAAAAAEI/YQaKo2zPyok/s800/2012-09-08%252020.20.15.jpg