December larger than 21, and a Opponent of Pittsburgh penguins had what average game?
CREATE TABLE "schedule_and_results" (
  "game" real,
  "december" real,
  "opponent" text,
  "score" text,
  "record" text
);
SELECT AVG("game") FROM "schedule_and_results" WHERE "december">21 AND "opponent"='pittsburgh penguins';