Question 667233
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Hi,
Suppose the measure of an exterior of an equiangular triangle is (x + 15)°.    What is the value of x?
Measure of an Interior angle of an equiangular triangle is: 60°
The exterior angle (x+15)° is the supplement of the interior angle:
 (x+15)° = 180° - 60° = 120°
   x+15°=120°
     x = 105°

Using Your method of solving:
Perhaps it was because of Your following statement did not say what you wanted it to say.
"Because the 2 angles of a triangle equals the measure of an exterior angle"

better stated would be something like this:
An measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles.