The triangle is an isosceles triangle with exactly two congruent sides. The two congruent sides are the radii. So that missing angle of the triangle on the very left is 'a' degrees as well
The three angles ('a', 'a' and 59) add up to 180 degrees. This is true for any triangle. Let's use this fact to solve for 'a'
a+a+59 = 180
2a+59 = 180
2a+59-59 = 180-59
2a = 121
2a/2 = 121/2
a = 60.5