CREATE TABLE table_name_87 (founded NUMBER, coach VARCHAR2, location VARCHAR2)

What is the average year that the club located in enfield was founded with an unknown coach?
SELECT AVG(founded) FROM table_name_87 WHERE coach = "unknown" AND location = "enfield"
SELECT AVG("founded") FROM "table_name_87" WHERE "coach" = "unknown" AND "enfield" = "location"