The applicable theorem is the one that tells you the exterior angle is equal to the sum of the opposite interior angles.
9x = (5x) +(9 +x)
9x = 6x +9 . . . . . . . . collect terms
3x = 9 . . . . . . . . . . . . subtract 6x
x = 3 . . . . . . . . . . . . . divide by 3
The value of x is 3.
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The angles (clockwise from left) are then
(9*3)° = 27°
(5*3)° = 15°
(9 +3)° = 12°
This is an obtuse triangle, obviously not drawn to scale.
