When the three sides of a triangle is known and you are looking for one of the angles, you make use of the cosine formular which states that [tex]\cos A= \frac{ b^{2} + c^{2} - a^{2} }{2bc} [/tex]; where A is the angle you are looking for (i.e. in this case x), a is the side opposite the angle you are looking for and b & c are the other sides of the triangle.
Therefore, [tex]\cos x^o= \frac{ 36^{2} + 59^{2} - 39^{2} }{2 \times 36 \times 59}= \frac{3,256}{4,248} =0.7665 \\ x^o=\cos^{-1}0.7665=40^o[/tex]
