user: Question: Which Opponent has a Game site of Heinz Field, and a Record of 3–1?
Columns: ['Week', 'Date', 'Kickoff (ET)', 'Opponent', 'Result', 'Record', 'Game site', 'NFL Recap']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Sun. Sep. 12', '1:00pm', 'Oakland Raiders', 'W 24–21', '1–0', 'Heinz Field', 'Recap']
SQL:

assistant: SELECT "Opponent" FROM "Table" WHERE "Game site" = 'Heinz Field' AND "Record" = '3–1';