m∠3 = 108°.
Since ∠1 and ∠2 are a linear pair, their sum is 180°. This means that
(5y-8)+(12+6y) = 180
Combine like terms:
11y+4 = 180
Subtract 4 from both sides:
11y+4-4=180-4
11y=176
Divide both sides by 11:
11y/11 = 176/11
y=16
We know that ∠3 and ∠2 are vertical angles; this means they are congruent. Substitute 16 for y in ∠2:
12+6y
12+6(16) = 12+96 = 108°
