So we are looking at three times the hypotenuse of a right triangle with an acute angle of 30 degrees, and the longer leg 25'. Each guy wire is therefore 25'/cos(30)=28.8675' Most antenna have (minimum) three guy wires for stability, so total length L=3*28.8675=866.03 m.
If the diagram shows other than 3 guy wires, multiply 28.8675 by the appropriate number accordingly.
