Get some drywall compound and drywall tape. Apply some drywall compound as to cover both 2xs and some of the wall next to it. Apply 2 strips of drywall tape to cover both joints and remove the excess with a taping knife. Let dry for a day or two. Apply another coat of drywall compound to smooth things out. Sand and paint.

The reason you're applying the drywall tape is because the joints will crack and be visible through the paint. The tape stops this from happening.

Or you can just apply drywall compound, sand and paint. However it will probably crack in the joints.