When you get tired of play 9th and 7th chords you can use this one! It’s technically an inversion and the root note doesn’t even appear in the chord form:
To locate it you’ll have to imagine where the root note would be if we played it (5th string). Look at the A6add9. The root would be the A string played open:
Since it’s so tricky, take special care to practice naming the chords on the 6/9 chords: