(x - h)2 + (y - k)2 = r2
where h is the x coordinate of the center of the circle.
where k is the y coordinate of the center of the circle.
where x is the x coordinate of point (x,y) on the edge of the circle.
where y is the y coordinate of point (x,y) on the edge of the circle.
Additional assistance here:
http://www.mathwarehouse.com/geometry/circle/equation-of-a-circle.php