smallest triangle 5.7/11,
medium triangle 11/x
5.7/11=11/x
x=11*11/5.7≈21.2 (longer leg of the medium triangle)
difference between 6 and 5.7 is 6-5.7=0.3 ,
screen going to be 0.3 feet less, than leg of the medium triangle that was found
21.2-0.3=20.9 ft