To answer this question, you need to know the formula to count volume of a cylinder. In this case, you are given the cylinder radius(6ft) and cylinder length/height(12ft). The formula should be:
Cylinder volume = 2*3.14 *radius*radius * length or width= 2 *3.14 * 6ft * 6ft * 12ft= 2712.96 ft cubic or 2713 ft cubic rounded