To determine the length of the unknown side of a right triangle, you typically use the Pythagorean theorem, which states:
\[ a^2 + b^2 = c^2 \]
where \( c \) is the length of the hypotenuse and \( a \) and \( b \) are the lengths of the other two sides.
Since you haven't provided specific information about the sides of the triangle, including which sides are known, it's not possible to definitively choose one of the given options (9 ft, 3 ft, \( \sqrt{65} \) ft, \( \sqrt{57} \) ft) without additional context.
If you can provide the lengths of the two known sides or any other information about the triangle, I'd be happy to help you determine the unknown side!