Let the circle is x2+y2+2gx+2fy+c=0,
having centre (−g,−f), since it passes through the point (3,4)
So, 9+16+6g+8f+c=0 ...(i)
And circle is intersecting the other circle
So x2+y2=36 orthogonally, so 2g(0)+2f(0)=c−36 ⇒C=36 ...(ii)
From Eqs. (i) and (ii) −6g−8f=61
Now, on taking locus of point (−g,−f),
we are getting 6x+8y−61=0