determining a profit function
I am stuck on this, i thought this is the right method used. Can anyone help me see what i did wrong?
Terry makes and sells necklaces. He has observed over time that when the price is $ 9 each, he sells an average of 16 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 $7. Express his profit P as a function of x, the number of necklaces sold.
What i did was:
P(x) = (9+x)(16-2x) - 7(16-2x)
144 - 18x + 16x -2x^2 -112 + 14x