Here is one way:
tan a = opp/ adj for Brett's pictured triangle ...where 'a' is the end of his shadow
= 1.85 / .6
arctan(1.85/.6) = 72 degrees
Now use this angle in the tower diagram
cos a = adj/hyp = 9.00/guywire
cos 72 = 9/guywire
guywire = 9 / (cos 72) = 29.17 m