Step-by-step explanation:
the supplementary angle to 139° (left of that angle, still under the red l line) is 180 - 139 = 41°.
therefore,
A = 180 - 90 - 41 = 49°.
B is the supplementary angle to A.
B = 180 - 49 = 131°
remember, the sum of all angles around one point on one side of a line is 180° (that is called supplementary angles).
and the sum of all angles in a triangle is always 180° too.
