All points on a circle has the same distance (radius) to the center. Specifically, the distance from point (x,y)to center (a,b) is \sqrt((x-a)^2+(y-b^2)), which is equal to radius m. So \sqrt((x-a)^2+(y-b^2))=m. Square both sides, (x-a)^2+(y-b)^2=m^2.
