Hello !Ankit jha 

Assume -Q is charge placed at centre of square which keeps system in equilibrium.Find total electrostatic force at centre due to 4 charges at corner and charge -Q.Now add vectorially all forces and equate to zero as system should be in equilibrium as per as questions.This expressions will give value of charge Q.

Hints:Length of daignol is (2½a).Charge will placed at length  ( a/2½) from corner of square.where a is length of side of square.

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