What team was promoted in the Serbian League East in the same season when Kolubara was promoted in the Serbian League Belgrade?
CREATE TABLE "table1_12283621_6" (
  "season" text,
  "serbian_league_belgrade" text,
  "serbian_league_east" text,
  "serbian_league_vojvodina" text,
  "serbian_league_west" text
);
SELECT "serbian_league_east" FROM "table1_12283621_6" WHERE "serbian_league_belgrade"='Kolubara';