The Law of Cosines features the 3 side lengths of a triangle, plus the measure of the angle opposite one of those sides.
We want angle x, which is opposite the side of length 39.
Then: a^2 = b^2 - 2ab cos C becomes 39^2 = 36^2 + 59^2 - 2(36)(59)cos x
or 1521 = 3481 + 1296 - 2(36)(59) cos x
Subtract (3481+1296) from both sides: 1521 - 4777 = -4248cos x
-3256 = -4248cos x
-3256
Then: cosx = --------------- = 0.766
-4248
Solving for x: x = arccos -0.766 = 0.698 radian, or 40 degrees (answer)