This is right angle trig. We know that...
cos(18°) x hypotenuse = 100
hypotenuse = 100/cos(18°)
hypotenuse = 105.15 meters approx.
Because they want the height of the tree we want "sin(18°) x hypotenuse".
sin(18°) x 105.15 = 32.5 meters approx.
answer: 32.5 meters approx.