Please help!?

Question

The total profit (in dollars) for sales of x rowing machines is given by P(x) = -0.2x^2 + 300x – 200. What is the profit if 500 are sold? For what value of x will the profit be at maximum?

Answer 1

P(500) = -0.2 * 50^2 + 300 * 50 - 200 = 14,300

P' = -0.4x +300 = 0 (at maximum profit), therefore
0.4x = 300
x = 750