Profit = Revenue - Cost
Revenue = R = px = -0.0006x^3 + 55x
Cost = C =-0.004x^2 +14x + 4000
Profit = R-C = -0.0006x^3 +0.004x^2 + 41x -4000
take the derivative and set =0
(R-C)' = -0.0018x^2 + 0.008x + 41 = 0
18x^2 - 80x - 410,000 = 0
9x^2 -40x -205,000=0
x = 40/18 +(1/18)sqr(40^2 +4(9)(205,000))
= (2716.91001+40)/18
= 153.1617
=153 rounded off to nearest integer