Answer:
86.63 ft
Step-by-step explanation:
The tangent relation applies.
Tan = Opposite/Adjacent
tan(77°) = (tree height)/(20 ft)
tree height = (20 ft)·tan(77°) ≈ 86.63 ft
_____
<em>Additional comment</em>
It is somewhat problematic for Bob to see the top of a white pine tree at that angle of elevation. His view would likely be obscured by the branches of the tree.
