The formula for the circumference of a circle is 2πr, where r is the radius, or πd, where d is the diameter.
I’m assuming they want the diameter in cm which is simple enough. 1 inch ≈ 2.56cm so 12 inches ≈ 30.72cm.
So using the formula from before, πd, we get the answer 30.72π.
If they want the diameter in inches then it is simply 12π but this is unlikely.
Hope this helps