user: Question: Who were the writers of the episodes directed by Greg Sullivan and which first aired on July 21, 2012?
Columns: ['Series #', 'Production Code', 'Title', 'Directed by', 'Written by', 'United States Original Airdate', 'Canada Original Airdate']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [27.0, 201.0, '"Zipper the Zoomit Dog"', 'Jos Humphrey', 'Mark Drop', 'June 2, 2012', 'September 7, 2012']
SQL:

assistant: SELECT "Written by" FROM "Table" WHERE "Directed by" = 'Greg Sullivan' AND "United States Original Airdate" = 'July 21, 2012';