Been on this problem for over an hour please help,

I keep getting my derivative at -.12x^2-10.8x-1200

with zeros at -154.659, 64.659 I dont know if thats right though

here is the question:

IfC(x) = 13000 + 400x- 0.6x2 + 0.004x3 is the cost function andp(x) = 1600 - 6xis the demand function, find the production level that will maximize profit. (Hint: If the profit is maximized, then the marginal revenue equals the marginal cost.)