Answer:
Door 3 diagonal 3√10 feet
Explanation:
The diagonal of a rectangle is the hypotenuse of a right triangle whose legs are the side lengths of the rectangle. That is, the diagonal of a 9 ft by 3 ft rectangle will have length ...
d = √(9² +3²) = √90 = 3√10
Any "9 ft by 3 ft" figure that has a diagonal different from this is not a rectangle. To fit the rectangular opening, we want a rectangular door—one with a diagonal of 3√10 feet.