A rule from geometry says that three points determine a circle.
Make a triangle with the three points. The perpendicular bisectors of the three sides will meet at the center of the circle. (Of course, you'll only need two but the third is confirmation.)
The points are A( 20, –18 ); B( 8, 18 ); C( –16, –6)
Graph paper, pencil and straightedge only.