With Sheffield United as the away team and the date, what home team has a tie no of 15?
CREATE TABLE "fourth_round_proper" (
  "tie_no" text,
  "home_team" text,
  "score" text,
  "away_team" text,
  "date" text
);
SELECT "home_team" FROM "fourth_round_proper" WHERE "tie_no"='15' AND "away_team"='sheffield united' AND "date"='sheffield united';