ECONOMICS - COST AND PRODUCTION... please?

Question

Mary's Fence Post Factory faces a perfectly elastic demand curve for fence posts at a price of $39 per post. Let Q represent the number of fence posts that Mary makes. Mary's total cost and marginal cost curves for making fence posts are:

TC = 4,000 + 3Q + 0.1Q2

MC = 3 + 0.2Q

2.3. If Mary maximizes her short-run profits, how much profit does Mary earn?

Answer 1

In perfect competition, to maximize profits, P=MC

39=3+.2Q

36=.2Q

Q=180

To find profits, find revenue minus costs

revenue=P*Q

39*180=7020

use total cost function for costs

4000+3(180)+.1(180)^2=

4000+540+3240=7780

7020-7780=-760