Answer-
The missing length to the nearest tenth is 20.2 ft.
Solution-
As given in the question, that the given triangle is a right angle triangle.
Pythagoras Theorem-

Here,
Hypotenuse = x ft
Base = 17 ft
Perpendicular = 11 ft
Putting the values,





Therefore, the missing length to the nearest tenth is 20.2 ft.