CREATE TABLE table_29087004_3 (written_by VARCHAR(255), directed_by VARCHAR(255), united_states_original_airdate VARCHAR(255))
Who were the writers of the episodes directed by Greg Sullivan and which first aired on July 21, 2012?
    
SQL: SELECT written_by FROM table_29087004_3 WHERE directed_by = "Greg Sullivan" AND united_states_original_airdate = "July 21, 2012"