Ab9/A# (Ab Dominant 9th over A#)

A slash chord names two things at once. The part before the slash is the chord your right hand plays, and the part after it is the single bass note for your left hand. Read Ab9/A# as Ab Dominant 9th with A# in the bass.

The bass note here is one of the chord tones, so this is exactly the 4th inversion of Ab Dominant 9th, written the way sheet music and chord charts usually write it.