user: Question: What year was the aircraft retired when it was Replaced by 737-300s?
Columns: ['Aircraft', 'Introduced', 'Retired', 'Seating', 'Notes']
Types: ['text', 'real', 'text', 'text', 'text']
Sample row: ['Airbus A319-100', 2004.0, '—', '156', 'In service']
SQL:

assistant: SELECT "Retired" FROM "Table" WHERE "Notes" = 'Replaced by 737-300s';