The distance of the observer and height of the building form a right angle triangle. Using:
tan∅ = height/base
We calculate the height of the building:
height = 64tan(39)
height = 51.8 meters
The second angle is obtained when the height of both the building and tower are used:
Height(building + tower) = 64tan(48)
51.8 + tower height = 71.1
tower height = 19.3 meters