The correct answer is x = 12
This is not correct. There are 180° in a triangle, not in each angle. To be 180°, the angle would need to be completely straight (not a triangle).
Since we know this is an equilateral triangle (all three sides are congruent to one another), it is also equiangular (all three angles are congruent to one another). There are 180° in a triangle -- which has 3 angles -- so divide that by 3 to get the measure of the angle (60°).
5x = 60
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5 5
x = 12