The angle of depression is the angle below the horizontal line from the air balloon and above the observers line of sight to the building. To solve, use
tanØ= opp/adj
opp: 50
adj: 165
This becomes
tanØ = 50/165
Ø = arctan(50/165)
Ø = 16.85°
This is only the angle between the balloons line of sight to the building and the vertical line it makes with the ground.
To get the angle of depression, subtract the angle you just found from 90° (the angles are complementary):
90°-Ø
90°-arctan(50/165) = 73.14°
or
90°-16.85° = 73.14°
The angle of depression is 73.14°