Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_72751 ( "Rank" real, "City 1" text, "City 2" text, "2012 Passengers (in millions)" text, "2011 Passengers (in millions)" text, "Distance" text )

### Question ###
How many passengers (in millions) in 2011 flew through along the route that had 6.640 million passengers in 2012?

### Accurate SQL ###

SELECT "2011 Passengers (in millions)" FROM table_72751 WHERE "2012 Passengers (in millions)" = '6.640'