Answer: the height of the tree would be 10 feet.
Explanation:
A tree and its shadow is proportional to Harry and his shadow. This means that
Height of the tree/height of the tree's shadow = Harry's height/height of his shadow
If Harry is 6 feet tall, his shadow is 9 feet and the tree's shadow is 15 feet long, then the height of the tree would be
6/9 = height of tree/15
height of tree = (6 × 15)/9 = 10 feet.