Question

What is the circumference of a circle with a diameter 9cm

Answer 1

The circumference follows the formula
C= 2 (pi)R
R is the radius which is half the diameter when we put this into the equation we get
C = 2 (pi)(4.5)
C=28.274334

Answer 2

Circumference is basically pi x diameter or pi x 2(radius)
in this situation it would be easier to multiple pi by 9 . 
this would tell you what the circumference is .
pi x 9 = 28.2743338823

hope this helps you