Table: serbian_league_teams
Columns: season
serbian_league_belgrade
serbian_league_east
serbian_league_vojvodina
serbian_league_west

Question: What team was promoted in the Serbian League East in the same season when Kolubara was promoted in the Serbian League Belgrade?
SQL: SELECT "serbian_league_east" FROM serbian_league_teams WHERE "serbian_league_belgrade" = 'Kolubara'