From what I can pick up from your problem statement, you want to know "a similar charm has a diameter of 4 cm what is the circumference." Please, would you use proper punctuation in your writing, for greater clarity:
"A similar charm has a diameter of 4 cm. What is the circumference?"
The formula for the circumference of a circle, when the diameter is known, is
C=[tex] \pi [/tex]d or C=pi*d
In this case, with diamter 4 cm, the circumference, C, is 4pi cm.
