The only solution is (4, -1/2).
The first one is wrong, because of you substitute X and Y or in this case (12, -5/2) into 3x+2y, you get 31=11, Which is an untrue mathmatical statement.
The second one is right, for the same reason. If you substitute X and Y you will get 11=11. which is a true statement.
The third one is wrong, because you have found a true statement.
The fourth one is wrong because you found an untrue statement.
