The illustration of the cylinder is as shown in the picture. Remember that a gas phase has molecules that are far away from each other. Because of this, they don't have a definite shape or volume. They only take the shape of its container. Therefore, the volume of the gas will be equal to the volume of the cylinder. The equation for determining its volumes is
V = π*r^2*h, where r is the radius of the cylinder and h is the height.
The radius is half of the diameter. Therefore, r = 6/2 = 3 ft. Substituting the values,
V = π*(3^2)*9
V = 81π or 254.47 cubic feet