The topic/ search query is "intercepted arcs"
The outermost angel is your far arc (135°)
The closer of the two on the circle is your near arc (missing variable "x")
The angle provided is your reference angle (39°)
The formula used to solve is far arc - near arc ÷ 2 = reference angle
So...algebra wise you'll want to reverse this equation in order to get your missing angle. Like so.
39 x 2 = 78, 135 - x = 78 once again, "x" is your variable / near arc
Therefore your answer for the missing degree is 57°