CREATE TABLE table_73412 ("Season no." FLOAT, "Series no." FLOAT, "Title" TEXT, "Directed by" TEXT, "Written by" TEXT, "Original air date" TEXT, "Production code" FLOAT, "U.S. Viewers  ( millions ) " TEXT)

who are the writer of the series episode number 170?
SELECT "Written by" FROM table_73412 WHERE "Series no." = '170'
SELECT "Written by" FROM "table_73412" WHERE "Series no." = '170'