CREATE TABLE table_name_30 (year_began_making_autos NUMBER, year_joined_gm VARCHAR2)

What is the average year to begin making autos for a brand that joined GM in 1917?
SELECT AVG(year_began_making_autos) FROM table_name_30 WHERE year_joined_gm = 1917
SELECT AVG("year_began_making_autos") FROM "table_name_30" WHERE "year_joined_gm" = 1917