Equal charges of value +q are placed of the orders A,B,C of a square ABCD having size of length l. At the corner D, there is a point charge -2q.
what is the electric field strength at the centre of the square
Easy. Corner opposite D has a charge of q, so net field inbetween is from D excess -q. The two charges on the adjacent corners, q at each, have fields equal and opposite in direction, so cancel.
Net field is due to - q at D, or E=k(-q)/distance where distance= .707 l