Marginal Profit Help: Find Production Level of 3000 Units per Month

[scienceguy12]

Homework Statement

An electronics manufacturing company has determined that the cost of producing x units
of its newest personal stereo is given by C = 4000 + 9.55x, and the monthly demand
equation is given by px=20,000 - x/500. Find the marginal profit for a production level of
3000 units per month.

I know the answer is $18.45 but I can't figure out how to start. Would I take the deriv of C? Surely not...

Homework Equations

The Attempt at a Solution

I know the answer is $18.45 but I can't figure out how to start. Would I take the deriv of C? Surely not...

 

[QuarkCharmer]

Edit: I misread the question.