The formula for the circumference of a circle is pi * r * 2
First we solve for r (the radius)
pi*r*2 = C
pi*r*2 = 18
Divide 18 by 2
pi*r = 9
Then divide 9 by 3.14
pi= 9/3.14 which is 2.866
The diameter is 2 times length of the radius so multiply 2.866 by 2
The diameter is about 5.732
(And you can round of course)
Hope that helped