CREATE TABLE table_name_81 (attendance INT, opponent VARCHAR, record VARCHAR)

What was the average attendance when the New York Mets were opponents with a record of 51-33?
SELECT AVG(attendance) FROM table_name_81 WHERE opponent = "new york mets" AND record = "51-33"
SELECT AVG("attendance") FROM "table_name_81" WHERE "51-33" = "record" AND "new york mets" = "opponent"