CREATE TABLE table_61488 ("Clubs" TEXT, "Seasons at this level" TEXT, "Settlements" TEXT, "Season joined league" TEXT, "Position in 1959\\u20131960" TEXT)

What is the 'season joined league' for the club that had 15 seasons at this level?
SELECT "Season joined league" FROM table_61488 WHERE "Seasons at this level" = '15 seasons'
SELECT "Season joined league" FROM "table_61488" WHERE "Seasons at this level" = '15 seasons'