Your getting warmer. More than likely the pipe going to the wall is 1 1/2 schd 40. Might be 2 inch but look on the pipe, it's usually stamped on it somewhere. You'll need to cut that short section between the last two glued pieces but leave it as long as possible so you have a stub to glue on the new adapter. Make sure the adapter accepts 11/2 slip joint size, you can always use an 1 1/4 washer in it if needed. Look the glue cans over before you buy, there are lots of choices.