Abadd9/D# (Ab Add9 over D#)

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 Abadd9/D# as Ab Add9 with D# in the bass.

The bass note here is one of the chord tones, so this is exactly the 2nd inversion of Ab Add9, written the way sheet music and chord charts usually write it.