Suppose That Dent Carr's Long-Run Total Cost of Repairing S

Suppose that Dent Carr's long-run total cost of repairing s cars per week is c(s) = 3s2 + 75. If the price he receives for repairing a car is $18, then in the long run, how many cars will he fix per week if he maximizes profits?

A) 3
B) 0
C) 6
D) 4.50
E) 9