The charge q1&q2 placed at corner of a square find q2 I.e resultant force on q1 is zero

U  would need to mention first the 4 charges alignment as U have ,mentioned that the charges are lying on the square. As it is unclear from the question that the charges are alignes diagonally otr at alternate position.

Since the net charge would be zero on q2 by q1 the force woud come zero.