If two electric charges q and −2q are placed at distance 6a apart, then locus of point in the plane of charges, where electric potential is zero is: (Take charge q at origin and −2q lies on positive x-axis)
kq√x2+y2+−2q√(x−6a)2+y2=0 (sum of forces has to be zero)
⇒ (x−6a)2+y2=4(x2+y2)
⇒ x2+y2+4ax−12a2=0