How to find P(x) given C(x)

Problem: Suppose C(x)= .02x^3 -.8x^2 +15x
a) Find marginal cost at x=10 ----> 5/item
b) If each item sells for $20, find profit function

I need the answer to b)

P(x) = R(x) - C(x) = 20x - (0.02x^3 - 0.8x^2 + 15x) = -0.02x^3 + 0.8x^2 + 5x