Go back to your basic definition:
We can get two points on this line: say (6, 1) and (6, 2).
Since we can't divide by 0, this slope is undefined.
Comment:
You have now posted several extra credit questions. I have noticed that many of the solutions depend directly on basic definitions. If the definitions are giving you problems I would strongly recommend you review these with your teacher to make sure you understand them. Otherwise, as I think Aftershock mentioned, none of this will help you.
-Dan