finding a 3 point circe

Hi, I need a good algorithem for finding the center and radius of a 3 point circle.

maybe something like:
radius(x1#,y1#,x2#,y2#,z3#,y3#)
center(x1#,y1#,x2#,y2#,z3#,y3#)

I know you have to connect the points and find the perpindicular linear intersections....that's all I know really.
Thanks in advance.