user: Question: Name the start for finish being 21st
Columns: ['Year', 'Chassis', 'Engine', 'Start', 'Finish']
Types: ['real', 'text', 'text', 'text', 'text']
Sample row: [1976.0, 'Eagle', 'Offy', '17th', '18th']
SQL:

assistant: SELECT "Start" FROM "Table" WHERE "Finish" = '21st';