Jeffrey is buying mulch for a circular flower garden with a diameter of 12 feet. the layer of mulch needs to be 4 inches thick. what is the volume of mulch needed for the flower garden?

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