Table schema:
CREATE TABLE table_29087004_3 (written_by VARCHAR, directed_by VARCHAR, united_states_original_airdate VARCHAR)

Question: 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"