Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_55242 ( "Brand" text, "Year founded" real, "Year began making autos" real, "Year joined GM" real, "Markets served today" text )

### Question ###
What is the average year to begin making autos for a brand that joined GM in 1917?

### Accurate SQL ###

SELECT AVG("Year began making autos") FROM table_55242 WHERE "Year joined GM" = '1917'