Answer:

In order to understand better this problem, we can use the figure attached, in which we have a right triangle (with a 90 degree angle), where
is its hypotenuse,
is the adjacent side to the angle of elevation
from the boat to the top of the lighthouse and
is the opposite side to
.
Knowing this, we will use the tangent trigonometric function to find
:
(1)
(2)
(3)
(4)
Finally: