Answer: 9 feet.
Explanation:
Given: A farmer needs to enclose a section of land in the shape of a parallelogram by using one side of a barn for one side.
We know that in the opposite sides of parallelogram are equal.
Since, it is given that the barn and the side opposite of the barn will both be 13 feet long and one of the other sides will be 9 feet long.
Therefore, the last side must be opposite to 9 feet long side.
Hence, the length of the last side must be 9 feet.