You can consider the tree perpendicular to its shadow, therefore you get a triangle rectangle of which you know the two legs and you need to find the angle between the shadow and the hypothenuse, opposite to the tree.
In this case, the angle α can be easily found with trigonometry:
tan α = (opposite side / adjacent side)
Therefore:
α = tan⁻¹ (tree / shadow) = tan⁻¹ (10 / 14) = 35°
NOTE:
tan⁻¹ is sometimes found written as arctan.
