What is the highest production code when the writer was brian egeston after season 5?
CREATE TABLE "season_6_2009_11" (
  "series_num" real,
  "season_num" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" real
);
SELECT MAX("production_code") FROM "season_6_2009_11" WHERE "written_by"='brian egeston' AND "season_num">5;