This is a volume of a cylinder question, so I wonder why you dismiss that. The equation is pi x radius^2 multiplied by height, which indeed comes to 81pi, which is somewhere in the neighborhood of 250 pounds per cubic foot.
You would need to use the formula for the volume of cylinder in order to get this. That's how I got to the answer of 81 pi. Maybe you used a different/wrong formula but you do need the formula for the volume of a cylinder.
