Trevor estimated that the building is 24 feet tal and its shadow is 18 feet long, therefore, we can see this as a right triangle. And we also have a pine tree whose shadow is 48 feet long.
In this cases, we consider the angles are the same, therefore, using trigonometry we can calculate the height of the tree as it follows
where H is the height of the tree, thus:
So, the big pine tree is about 64 feet tall,