The plane figure formed by the ground, the guy wire, and the tree is a right triangle with the hypotenuse equal to length of the guy wire. The angle given is an angle adjacent to 3.5 ft. Therefore, the most suitable trigonometric function for this is,
cos (50°) = adjacent / hypotenuse
cos 50° = 3.5 ft / hypotenuse
The value of the hypotenuse is 5.445 ft.
Hence, the length of the guy wire is approximately 5.445 ft.