What is the average speed for ships before 1974 with over 1.73 passengers?
CREATE TABLE "ro_ro_passenger_ships" (
  "ship_name" text,
  "year" real,
  "length" text,
  "width" text,
  "passengers" real,
  "vessels" real,
  "speed" real
);
SELECT AVG("speed") FROM "ro_ro_passenger_ships" WHERE "year"<1974 AND "passengers">1.73;