12. x = 5, m∠1 = 13°13. a = 9°
Step-by-step explanation:
12
Given:
m∠1 = (2x + 3) m∠3  =  (6x - 17) x? m∠1?
Solution:
∠1 and ∠3 are vertical angles.
We know the property that vertical angles are equal:
2x + 3 = 6x - 176x - 2x = 3 + 174x = 20x = 5m∠1 = 2*5 + 3 = 13°13
Given
m ∠2 =  (15a + 8) m ∠3 =  (5a - 8)a = ?
Solution
∠2 and ∠3 are supplementary angles.
We know the property that supplementary angles add up to 180°:
15a + 8 + 5a - 8 = 180°20a = 180°a = 180°/20a = 9°