When 656000 is the amount of viewers and n/a is the cable rank what is the airdate?
CREATE TABLE "series_6" (
  "episode_no" real,
  "airdate" text,
  "viewers" real,
  "bbc_three_weekly_ranking" text,
  "cable_rank" text
);
SELECT "airdate" FROM "series_6" WHERE "cable_rank"='N/A' AND "viewers"=656000;