Table schema:
CREATE TABLE table_name_77 (engine_type VARCHAR, numbers VARCHAR, length VARCHAR, year VARCHAR)

Question: 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)"