math optimization problem?

Terry makes and sells necklaces. He has observed over time that when the price is $9 each, he sells an average of 24 per day. If he increases the price, then his average sales fall by 2 per day for each dollar increase. The materials for each necklace cost $4. Express his profit P as a function of x, the number of necklaces sold. I got P(x) = 30x - 1/3x^2 but it is wrong??