First, substitute the given radius and given height of the cone into the formula for the volume of a cone.
Volume of a cone: (1/3) pi r^2 h
In this case, the cone volume is (1/3) pi (12 cm)^2 (12 cm) = 576 cm^3
Vol. of a rt. cylinder: pi r^2 h
In this case, the vol. of the rt. cyl. is pi (8 cm)^2 h.
The two different shapes have the same volume. Therefore, set the two formulas (above) equal to each other:
576 pi cm^3 = pi (8cm)^2 h. This becomes
576 pi cm^3 = pi (64 cm^2) h. "pi" cancels out, leaving us with
576 cm^3
------------- = h. This is the height of the cylinder, in cm.
64 cm^2