The idea is to create a triangle based upon the info given (see attached drawing). The bottom side (c) represents the length of the tree's shadow cast by the sun (18 ft), whose opposite angle (C) = 68°, the angle of the sun's elevation. The left side (b) is the height of the tree. The top side (a) is not needed, but it's opposite angle (A) is on order to find out B.
Since the hill incline is 16°, the angle A (angle tree makes with the hill) is 90-16 = 74°. Since C = 68 and A = 74, then B = 180-68-74, because all three angles must equal 180° within a triangle. So B = 38°.
Now that we have two angles and a side of our triangle, we can use the law of sines to calculate a missing side (b).
Law of sines: a/sinA = b/sinB = c/sinC
--> b = c×sinB / sinC
b = 18×sin38 / sin68
b = 11.082 / 0.927 = 11.95 ft
