The triangles shown are similar triangles (Mirrored and Enlarged)
This problem is based of of page 498, problem number 1, in our Student Text Volume 1. In this problem, you use similar triangles and ratios to determine the missing side (the height of the tree).
To find the solution, you must create a ratio between the small triangle and the large one. Since you know the triangles are similar, you can see that the 4ft side corresponds to the 12ft side. You can divide 12 by 4 and get 3 as a result. Then, you take 5 times 3, and get 16.5 as your final answer!