What is the average number of points for an Offenhauser L4 engine with a Trevis chassis?
CREATE TABLE "complete_formula_one_world_championship_" (
  "year" real,
  "entrant" text,
  "chassis" text,
  "engine" text,
  "points" real
);
SELECT AVG("points") FROM "complete_formula_one_world_championship_" WHERE "engine"='offenhauser l4' AND "chassis"='trevis';