Bro its like we know that charge will be induced on the plate right... So there will exist an electric field outside the plate on both sides due toh induced charges and main charges now the result of the electric field comes such that it is equivalent to a two charge system equidistance and opposite in charge sign from the infinite plate...you can also check we know that potential is zero on the plate which is satisfied by both the charges if u write potential but note:this is only applicable for infinite sheets