Question 319230
If the rope is straight and not sagging, then
you can consider the height of the wall
one leg of a right triangle and
the distance from the stake to the wall as the 
other leg of the right triangle.
The length of the rope is the hypoteneuse
The pythagoran theorem is:
{{{h^2 = a^2 + b^2}}}
{{{39^2 = 15^2 + b^2}}}
{{{1521 = 225 + b^2}}}
{{{b^2 = 1296}}}
{{{b = 36}}}
The building is 36 feet tall