CREATE TABLE table_name_77 (engine_type VARCHAR(255), numbers VARCHAR(255), length VARCHAR(255), year VARCHAR(255))
If length is ft (m) with numbers of 6600-6684 (84 buses) for 2003, what is the engine type?
    
SQL: SELECT engine_type FROM table_name_77 WHERE length = "ft (m)" AND year = "2003" AND numbers = "6600-6684 (84 buses)"