If you were on a planet near Alpha Centauri and looked toward our sun, what constellation would it be in?

Alpha Centauri is at coordinates RA 14h 40m, Dec -60d 50', from our point of view. So looking from Alpha C, the coordinates of Earth are to opposite: 9h 20m, Dec +60d 50'. That's in the constellation Ursa Major (the Big Bear).

Unknown you have to be on that planet to figure it out.

Cassiopeia.

The constellations as we know them would not exist from a different POV in the Galaxy, as some stats are nearer and some farther away and their angular relationships would change from another location------its not like the constellations are on a flat backdrop to Earth-----they are the star configurations in our sightlines from Earth. These sightline would change from another position.------YES, Alpha Centuri is not all that far away, but still far enough away for the stars to look somewhat differently arranged.

Opposite would be Right Ascension 02h 40m, not 14h 20m because the sphere of stars is 24 hours around, so opposite will be 12 hours difference, not 5h 40m.

The Declination would be +60d 50'

The sky behind 02h 40m and +60d 50' is in between Cassiopeia, Perseus, and Camelopardalis, just north of the Double Cluster. That is what is shown on Google Sky map and Sky & Telescope Magazine center pages sky dome map.