user: Question: What is the length of the coaster with the unknown speed
Columns: ['Name', 'Park', 'Length (ft)', 'Height (ft)', 'Speed (mph)', 'Opened', 'Status']
Types: ['text', 'text', 'real', 'real', 'text', 'text', 'text']
Sample row: ['Boardwalk Bullet', 'Kemah Boardwalk', 3236.0, 96.0, '51', 'August 31, 2007', 'Operating']
SQL:

assistant: SELECT "Length (ft)" FROM "Table" WHERE "Speed (mph)" = 'unknown';