Answer: 
11.99 ft
Step-by-step explanation:
The formula for the volume of a cyl. of radius r and height h is V = πr²h.
Solving this immediately for h yields:
         V
h = ----------
         πr²
Inserting the known quantities results in:
         V           1356.48 ft³
h = ---------- = ----------------------      Note that r = (1/2)d, so r = (1/2)(12 ft) = 6 ft
         πr²         3.14159 (6 ft)²
   =   11.994 ft
The height of the cyl. is 11.99 ft (to the nearest hundredth foot)