Answer:
Step-by-step explanation:
You have to assume the the marked lines are parallel.
Alternate exterior angles are congruent, so ...
(15x -26)° = (12x +1)°
3x = 27 . . . . . . divide by °, add 26-12x
x = 9 . . . . . . . . .divide by 3
Then the alternate exterior angles are ...
(12·9 +1)° = 109°
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The angle at upper right is vertical with the obtuse interior angle of the triangle whose other interior angles are marked. The sum of them is 180°, so we have ...
28° +109° +(4y -9)° = 180°
4y = 52 . . . . . . divide by °, subtract 128
y = 13 . . . . . . . . divide by 4
Then the third interior angle of the triangle is ...
(4·13 -9)° = 43°
