Circumference has two formulas: C = pi times diameter and C = 2 times pi times the radius. In this problem you are given the radius. So, you need to fill in the blanks.
Step one set up your formula:
C= 2 times pi times radius
Step two fill in the blanks
C= 2 times (what you are using for pi) times 81
The answer is (using 22/7) 509.14
If this is not the answer you were looking for my apologies.
