What is the average week number for games that had 28,800 fans in attendance?
CREATE TABLE "regular_season" (
  "week" real,
  "date" text,
  "opponent" text,
  "location" text,
  "final_score" text,
  "attendance" text,
  "record" text
);
SELECT AVG("week") FROM "regular_season" WHERE "attendance"='28,800';