Hello,
Using Thalès 's theorem:
Let's h the height of the monument.
[tex]\dfrac{1}{h} =\dfrac{4.5}{78} \\\\h=\dfrac{78}{4.5} =\dfrac{156}{9} \approx{17.3\ (ft)}[/tex]
If you don't know what is the Thalès's theorem,
imagine that the similar triangles.
