What is the average year to begin making autos for a brand that joined GM in 1917?
CREATE TABLE "brand_reorganization" (
  "brand" text,
  "year_founded" real,
  "year_began_making_autos" real,
  "year_joined_gm" real,
  "markets_served_today" text
);
SELECT AVG("year_began_making_autos") FROM "brand_reorganization" WHERE "year_joined_gm"=1917;