CREATE TABLE table_31879 ("Played in" TEXT, "Matches" TEXT, "Lost" TEXT, "Drawn" TEXT, "Points for" TEXT, "Points against" TEXT)

The 51 points against, how many are for?
SELECT "Points for" FROM table_31879 WHERE "Points against" = '51'
SELECT "Points for" FROM "table_31879" WHERE "Points against" = '51'