what is the year when the location is yankee stadium and the result is 23-23
CREATE TABLE "1970s_eagles_15_4_1" (
  "year" real,
  "date" text,
  "winner" text,
  "result" text,
  "location" text
);
SELECT AVG("year") FROM "1970s_eagles_15_4_1" WHERE "location"='yankee stadium' AND "result"='23-23';