For this case we have the following function:
 P (x) = - 3 (x-5) ^ 2 + 12
 We derive the function to obtain the maximum.
 We have then:
 P (x) = - 6 (x-5)
 We match zero:
 -6 (x-5) = 0
 We clear x:
 x-5 = 0
 x = 5
 Answer:
 the company should set a sock price of 5 $ to earn a maximum profit