If length is ft (m) with numbers of 6600-6684 (84 buses) for 2003, what is the engine type?
CREATE TABLE "bus_fleet" (
  "year" text,
  "make_model" text,
  "length" text,
  "engine_type" text,
  "numbers" text
);
SELECT "engine_type" FROM "bus_fleet" WHERE "length"='ft (m)' AND "year"='2003' AND "numbers"='6600-6684 (84 buses)';