user: Question: What is Tries Against, when Tries For is 21?
Columns: ['Team', 'Tries for', 'Tries against', 'Try diff', 'Points for', 'Points against', 'Points diff']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Castres Olympique', '29', '6', '+23', '207', '71', '+136']
SQL:

assistant: SELECT "Tries against" FROM "Table" WHERE "Tries for" = '21';