Hope this helps?
Desmos is a great resource to help you understand the concept.
the radius: r^2 = 81
r =9
c = 2*pi*r
= 2 * pi * 9
= 18 *pi
Answer: 18*pi or approximately 56.62
It's not a function because the x value repeated but wasn't accompanied by the same y value . there is (-2,2) and (-2,14) but if they had the same y value like (-2,2) and (-2,2) or (-2,14) and (-2,14) the table would represent a function. If you still dont understand I'll be more than happy to elaborate :)