CREATE TABLE table_21912 ("Episode" CLOB, "Broadcast date" CLOB, "Run time" CLOB, "Viewers  ( in millions ) " CLOB, "Archive" CLOB)

Name the broadcast date for 6.4 million viewers for the archive of 16mm t/r
SELECT "Broadcast date" FROM table_21912 WHERE "Viewers (in millions)" = '6.4' AND "Archive" = '16mm t/r'
SELECT "Broadcast date" FROM "table_21912" WHERE "Archive" = '16mm t/r' AND "Viewers (in millions)" = '6.4'