CREATE TABLE table_24936 ("No. in series" FLOAT, "No. in season" FLOAT, "Title" CLOB, "Directed by" CLOB, "Written by" CLOB, "Original air date" CLOB, "Production code" CLOB, "U.S. viewers  ( million ) " CLOB)

Who wrote the episode that has a production code 3x7557?
SELECT "Written by" FROM table_24936 WHERE "Production code" = '3X7557'
SELECT "Written by" FROM "table_24936" WHERE "Production code" = '3X7557'