Question 875326
The shadow is on the ground that is expected to be horizontal.
The height of the object is measured as a vertical distance, from the top of the object, straight down to the ground.
{{{drawing(300,120,-0.5,14.5,-0.5,5.5,
arrow(-1,0,18,0),blue(line(0,0,12,0)),
line(12,0,12,4),red(arrow(0,0,18,6)),
rectangle(12,0,11.5,0.5),
red(arrow(18,6,12,4)),
locate(5,0.8,blue(shadow)),
locate(12.1,2.3,height),
locate(12,5.5,red(sunbeam))
)}}}
The vertical height of the object and the horizontal shadow of the object are the legs of a right triangle. The hypotenuse of that right triangle is the line that connects the top of the object to the tip of the shadow, and is aligned with the beam of sunlight grazing the top of the object.
At a certain location and time, all sunbeams make the same angle with the ground,
so all right triangles formed by object heights and their corresponding shadows are similar right triangles.
The ratio of vertical leg (height) to horizontal leg (shadow length) is the same.
So, the length in feet {{{x}}} of the shadow of an object {{{12.3}}} feet tall and the object's height are in the ratio {{{x/12.3}}} , and that is the same {{{11.7/3.9}}} ratio as for the {{{3.9}}} feet tall object casting a {{{11.7}}} feet long shadow.
 
{{{x/12.3=11.7/3.9}}} ---> {{{x=12.3*11.7/3.9}}} ---> {{{highlight(x=36.9)}}}