Point charges — UPSC CSE 1989 Physics PYQ

Two point charges each of magnitude +2\text{ millicoulomb} are at A and B in front of a infinite conducting plane which is grounded. The line OAB is the perpendicular to time plane with the point O on the plane. If \mathrm{OA}=1\text{ m} and \mathrm{OB}=2\text{ m} calculate the force on the charge at A