The answer is 18 ft.
x - the height of the building.
x1 - the height of the tree.
y - the distance between the building and the stake.
y1 - the distance between the tree and the stake.
x = 36 ft
x1 = ?
y = 1
y1 = 0.5 (because it is halfway between the stake and the building, which is y)
By similar triangles:
x : x1 = y : y1
36 : x1 = 1 : 0.5
x1 = 36 * 0.5 : 1
x1 = 18 ft
