Given : Concentric circles with center(1,2)
The equation of concentric circles with centre(1,2) is
$(x-1)^2+(y-2)^2=a^2$
Now let us differentiate w.r.t x
$2(x-1)+2(y-2).\large\frac{dy}{dx}$$=0$
Dividing throughout by 2,
=>$(x-1)+(y-2)\large\frac{dy}{dx}$$=0$
This is the required equation