Let x = the distance between the two buildings
Let d = the angle of depression
Let y = the difference in height
In the attached diagram, the black lines represent the two buildings, and the red lines represent the triangle that is created when you connect the two buildings by an imaginary line.
The unknown value, y, can be solved using the trigpnometric function tangent:
Therefore, the difference in height between the tall building and the short building is 19.4 ft.