Math - Geometry: Cirference Equation needed?

Ok, well I found a page explaining this a long time ago and can't find it again, Internet Explorer History is deleted, for obvious reasons, hahaha. YANK! But anyways, It's about 2 polar coordinates on a circle, (-2,1) (8,3), and they are the hypotenuse of a triangle in the circle. Basically, they want the equation of the cirference. I know this is easy, but I am just hitting a block right now, and just getting really stupid lately. I have the radius through difference between 2 points, divided by 2 because they ARE polar... blah blah blah, Please help, and I promise I will stop doing things that lead to me deleting my History. Thanks for any help.