By the way, it's pi, not pie. just wanted you to know as you may lose marks in tests for spelling (but i may be wrong)
we know circumference is pi multiplied by the diameter.
the diameter is 2x the radius
we can re-arrange the first fact to get:
diameter (or 2x radius) = circumference divided by pi.
since the circumference is 24pi we can cancel off pi from the circumference (24pi) and pi.
now we have the diameter = 24
since the diameter is 2x the radius you divide both sides by 2 and get:
radius=12cm