i dont have an explanation but that is the answer i can confirm because i did it and got it wrong but then it showed the right answer hope it helps you
The difference of the length between the 2 buildings D=L1-L2 D=95-55 D=40 feet we use Pythagoras formula D^2+X^2=Z^2 X=distance between buildings Z=length of the zip line Z=50 feet