The equation to find a circumference is
(c = circumference, r = radius, d = diameter)
c = 2r · π
So we need to find the radius.
A radius is half of the diameter, so
12/2 = 6
Now put the value in the equation and solve.
c = 2 · 6 · π
c = 12 · π
c = 37.7
The answer is 37.7
Hope this helped! If you have anymore questions or don't understand, please comment or DM me. :)