Given that the circles x2+y2−2x+6y+6=0 and x2+y2−5x+6y+15=0 touch, the equation to their common tangent, is
x=3
y=6
7x−12y−21=0
7x+12y+21=0
Using S1−S2=0 WC obtain 3x−9=0 or, x=3 as the equation of the required common tangent