who is the featuring when the doctor is the 6th and the series sorted is 6y/ak?
CREATE TABLE "series_3" (
  "series_sorted" text,
  "title" text,
  "doctor" text,
  "featuring" text,
  "released" text
);
SELECT "featuring" FROM "series_3" WHERE "doctor"='6th' AND "series_sorted"='6y/ak';