user: Question: How many passengers (in millions) in 2011 flew through along the route that had 6.640 million passengers in 2012?
Columns: ['Rank', 'City 1', 'City 2', '2012 Passengers (in millions)', '2011 Passengers (in millions)', 'Distance']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Seoul', 'Jeju', '10.156', '9.9', '450km']
SQL:

assistant: SELECT "2011 Passengers (in millions)" FROM "Table" WHERE "2012 Passengers (in millions)" = '6.640';