Let's think of an example which would better help visualize this situation.
1) Someone exercises a lot so they can concentrate more on their homework (here running and concentration on homework are dependent events)
2) Someone exercises a lot because they wear a blue t-shirt (here, you can clearly see that wearing a blue shirt and exercising are not related. These events are independent).
Now concentrating on P(a | b) = Probability a occurred if b occurred.
It does not matter if b occurred (just like it didn't matter that the person wore a blue shirt which meant that they exercised) for the outcome a to occur.
We are given an angle of elevation of 2 degrees and distance in the x axis of 5280 feet and we are asked in the problem to determine the height of the building. We use the tangent function to determine the height: that is tan 2 = h / 5280; h is equal then to 184 ft.
