It doesn't say if you need your answer in inches or feet but either way you have to convert the diameter to inches or the thickness of mulch to feet.
I did it both ways.
v = pi r^2 h is the formula for volume of a circular cylinder. This is the shape you would end up with because of the thickness of the mulch.
In feet: v=pi(6^2)(1/3); v = pi(36)(1/3); v=pi(12) or 12pi ft^3; If you have to use 3.14 as pi then the answer would be 37.68 ft^3.
In inches: v=pi(72^2)(4); v=pi(5184)(4); v= pi(20,736) in^3; if you have to use 3.14 as pi then the answer would be v = 65,111.04 in^3
