The radical axis of the circles, belonging to the coaxal system of circles whose limiting points are

$(1, 3)$ and $(2 ,6)$, is  

We know that the limiting points are circles of the coaxial system with zero radius. The equations of the point circles having $(1, 3)$and $(2, 6)$ as limiting points are 

$S_1\equiv (x-1{)}^2+(y-3{)}^2=0$       (i)

and , $S_2\equiv (x-2{)}^2+(y-6{)}^2=0$      (ii)

The equation of their radical axis is 

$S_1-S_2=0\Rightarrow x+3y-15=0$