Formula for volume of a sphere: V = (4/3)pi*r^3
18 cm radius: VR = (4/3)pi*(18 cm)^3
12 cm radius: Vr = (4/3)pi*(12 cm)^3
Calculate both, and then subtract the 2nd from the first. I would take the short cut of finding the difference as follows:
Difference in volume = (4/3)(3.14)*[18^3 - 12^3] cm^3