user: ### NOW ANSWER:
Question: Where the length is 91', what is the year built?
Columns: ['Name', 'Type', 'Year Built', 'Where Built', 'Length']
Types: ['text', 'text', 'real', 'text', 'text']
Sample row: ['Messenger', 'sternwheeler', 1872.0, 'Empire City', "91'"]
SQL:

assistant: SELECT "Year Built" FROM "Table" WHERE "Length" = '91''';