The given circumference is C = 12*pi
The formula for the circumference of any circle, with radius r, is
C = 2*pi*r
Plug in the given value of C and solve for r
C = 2*pi*r
12*pi = 2*pi*r ... C has been replaced with 12*pi
12 = 2*r ... divide both sides by pi (they cancel out)
6 = r ... divide both sides by 2
So r = 6 is the radius