If you mean "find the circumference of a circle with a diameter of 18 mm" then:
Circumference = Pi * Diameter
C = 18 * 3.14 (approximate value for Pi)
C = 56.52 mm
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If you mean "find the circumference of a circle with an area of 18 mm^2" then:
Area = Pi * Radius^2
Area / Pi = Radius^2
18/3.14 = Radius^2
5.73 = Radius^2
2.39 = Radius
Circumference = 2 * Pi * Radius
Circumference = 2 * 3.14 * 2.39
Circumference = 15.01
The circumference should be about 15 mm.