how to you get the radius if you only have the circumference?

This is what Anonymous is saying in a more compact manner

C = 2*pi*r    divide by 2*pi on both sides

C/(2*pi) = r

(Don't worry, Anonymous, I'll give you the "points, here!!)

Since the equation for circumference is Pi times the diameter, and diameter is 2 radius, dividing the number you used for Pi then divide the number you got after by 2, you will find the radius

Anonymous doesn't get any credit for the points - although he/she can see them.

I think the verbal praise is more important to everyone but especially to the anonymous people.

You did good anonymous. Thank you