What is the average Week when there were more than 63,866 people in attendance on September 7, 1981?
CREATE TABLE "schedule" (
  "week" real,
  "date" text,
  "opponent" text,
  "result" text,
  "attendance" real
);
SELECT AVG("week") FROM "schedule" WHERE "attendance">'63,866' AND "date"='september 7, 1981';