The lines intersect to create a triangle. The sum of the interior angles of a triangle is 180 degrees.
a + b+ c = 180
We deduce the unlabeled angle (in the lower right corner) by considering the line that goes from that angle to the a angle on the left side. The line itself has a 180 degree angle (on each side). And we know the exterior angle is 107 degrees... that leaves 73 for the interior angle.
We take the equation above again... by putting now the value of C (the un-named angle):
a + b + 73 = 180
We isolate the a+b part... to get a+b = 107, the same value as the exterior angle.
