The formula for finding the circumference of a circle is 2πr or πd
So, there are 2 ways to find it.
1) Using the diameter, you would do π x 14.
This will give you 43.9822971503
(round the number to however many dp it says to do :) )
2) Using the radius (half of the diameter), you could do 2π x 7
which will also get you 43.9822971503