What is the national cup statistics when the Championship is 21 app / 6 goals?
CREATE TABLE "career_statistics" (
  "season" text,
  "club" text,
  "country" text,
  "championship" text,
  "national_cup" text,
  "international_cup" text
);
SELECT "national_cup" FROM "career_statistics" WHERE "championship"='21 app / 6 goals';