ACME Antenna Company builds cellphone antennas that sell for $200 per foot of height. Since taller antennas are more complicated to construct and support, the cost of constructing a portion of the antenna varies with its distance (height) from the base of the antenna. ACME has learned that their cost to produce a portion of the antenna that is x feet above the base is 3x dollars per foot.
(Note that profit is defined as the difference between the amount of money that ACME receives for selling the antenna and the company's cost to construct the antenna.)
