angle at a straight line is =180
4+5+x=180
let x be angle at the tower
9+x=180
x=180-9
x=171°
and x is divide in to two then is 2x
there for
2x=171°
x=171/2
x=85.5°
then sin85.5°= 90/y
let y be the unknown feet
0.9969=90/y
y0.9969=90
y=90/0.9969
y=90.3ft.