How do you find radius with given circumference?

You're supposed to know that

Circumference = (pi) x (diameter)

Divide each side of this equation by (pi) :

Diameter = Circumference/pi

And you also know that Diameter = 2 x Radius .

2 x Radius = Circumference/pi

Divide each side of that one by 2 :

Radius = Circumference/2 x pi .

and that's exactly what you need.