Answer:
Step-by-step explanation:
Given triangle is equilateral since all three angles marked as congruent.
It means each interior angle measures 60°.
The angle (5x + 15)° is exterior angle.
We know exterior angle measure is same as the sum of non-adjacent interior angles:
- 5x + 15 = 60 + 60
- 5x + 15 = 120
- 5x = 120 - 15
- 5x = 105
- x = 105/5
- x = 21
