C13b9/Db (C 13b9 over Db)

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 C13b9/Db as C 13b9 with Db in the bass.

The bass note here is one of the chord tones, so this is exactly the 4th inversion of C 13b9, written the way sheet music and chord charts usually write it.