What was the title of the episode with a production code of 3x6404?
CREATE TABLE "table1_27776266_1" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text,
  "u_s_viewers_million" text
);
SELECT "title" FROM "table1_27776266_1" WHERE "production_code"='3X6404';