potential at A

use the method of electrical image :
F= Kq² / (2r)²
now find dw= F.dx
...integrate dw with limits from r to ∞....

debo... what is the method of electric image?

it is used to calculate force of interaction between a source charge and an infinite conducting plane ! the source charge induces some charge on the plane and we use this method to calculate the force between the source charge and the induced charge !

but then answer is given as zero.. which I dont get using this method

yes,,,the answer is right ! the potential at any point on the perp bisector of the axis of a dipole is zero ~