Answer:
The 51.39 feet far is the boat from the lighthouse .
Explanation:
Now by using the trignometric identity.

As given
The angle of depression from the top of a lighthouse to a boat in the water is 30°.
If the lighthouse is 89 feet tall .
As shown in the figure given below.

Base = AC = 89 feet
Perpendicular = CB
Put in the identity .


Put in the above

CB = 51.39 feet (Approx)
Therefore the 51.39 feet far is the boat from the lighthouse .