The circumference of a circle is given by the formula:
[tex]C\text{ = 2 }\pi\text{ r}[/tex]The diameter of the circle, d = 18cm
Radius = diameter / 2
r = d/2
r = 18/2
r = 9 cm
[tex]\begin{gathered} C\text{ = 2}\pi(9) \\ C\text{ = 18}\pi \end{gathered}[/tex]