The formula for the circumference of a circle is:
C=2(pi)r
C is the circumference, pi is equal to around 3.14, and r is the radius.
In this problem, we do not have the radius but is given the diameter. You must understand that radius(2) is equal to the diameter and diameter/2 is equal to the radius. So to find the radius, we will divide the diameter by 2. 42/2=21. The radius is 21. Now we will put it in the formula.
C=2(pi)r
C=2(pi)21
C=131.95
Hope this helped