CREATE TABLE table_72751 ("Rank" FLOAT, "City 1" TEXT, "City 2" TEXT, "2012 Passengers  ( in millions ) " TEXT, "2011 Passengers  ( in millions ) " TEXT, "Distance" TEXT)

How many passengers (in millions) in 2011 flew through along the route that had 6.640 million passengers in 2012?
SELECT "2011 Passengers (in millions)" FROM table_72751 WHERE "2012 Passengers (in millions)" = '6.640'
SELECT "2011 Passengers (in millions)" FROM "table_72751" WHERE "2012 Passengers (in millions)" = '6.640'