CREATE TABLE table_29289372_1 (area__km²_ VARCHAR(255), income_classification VARCHAR(255), population__2010_ VARCHAR(255))
When 318676 is the population of 2010 and 1st class is the income classification what is the area in kilometers squared?
    
SQL: SELECT area__km²_ FROM table_29289372_1 WHERE income_classification = "1st Class" AND population__2010_ = 318676