Hey there!
Given that this flower bed is circular, it can be assumed that it's a cylinder. In this problem, we need to use the equation for the volume of a cylinder:
V = π r² h
Since we just need to find the area that the mulch will take up, we'll use that given measurement of 4 inches in place of the actual height of the flower garden. Since we need the radius, all we have to do is divide the 12 inch diameter measurement in half to get a radius of 6 inches. Usually for these problems, they'll tell you to use 3.14 for pi instead of the full pi number, so we'll just use that to make things easier.
V = π (6)² (4)
V = π (36)(4)
V = π (144)
V = 452.16
The volume of the mulch needed will be 452.16 in³.
Hope this helped you out! :-)