Answer:
I would measure a 45° angle from the ground to the building. Then I would back up until the angle aligns with both the ground and the top of the building. I would measure the distance between me and the building to get the height of the building. Since the triangle is a 45, 45, 90 triangle, the distance between me and the building would be the same as from the ground to the top of the building.
Explanation:
Sorry I'm not drawing a sketch for this but I'll put it very simply with a 45, 45, 90 triangle because that's easiest.