The measure of angle B is 119 degrees.
What is Exterior angle?
Exterior angle is an measure of rotation between one extended side(we extend it virtually) of the polygon with its adjacent side which is not extended.
All the exterior angles are of same measure, and therefore, their measure is (360/n)°.
The angle adjacent to 112° will form a straight angle along with 112° adding to 180° which makes angle C: 68°
Then;
a+c+51 = 180
a+68°+51° = 180°
a+119° = 180°
a = 180° - 119°
a = 61°
Now,
a+b = 180°
61+b = 180
b = 180-61
b = 119°
Learn more about interior and exterior angles here:
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