Theorem:
The segment that joins the midpoints of two sides of a triangle is half the length of the third side and parallel to the third side.
Look at the large triangle formed by the entire horizontal bottom side, the left side of the building, and the side that comes down from the top of the building to the ground on the left side.
The bottom side of the triangle is split into two congruent parts with two little marks in each. The bottom of the tree is at the midpoint of that side. The top of the tree is the midpoint of the top sloping side. That means that the tree is parallel to and half the length of the side of the building.
Answer: 60 ft