I presume from the thread title that is the determinant of the matrix . I'm afraid your arguments only allow you to say that if are matrices with and , then which is vacuously true. That does not imply . Example: .

You started at the right place: Since are nonzero, they are invertible. Then , so if we can show , then we have found our !

Now we need some properties of the determinant, namely that and . Then it is easy to see that as required.