Explanation:
a is the interior angle of the 2 opposing arc angles. the interior angle is the mean value between the 2 arc angles.
a = (145 + 45)/2 = 190/2 = 95°
b is the simply the supplementary angle to a (meaning together they are 180°, because the sum of all angles around a single point on one side of a line is 180°).
b = 180 - 95 = 85°