Looking at the given diagram, The angles formed have a common vertex. This means that they are vertically opposite angles. Vertically opposite angles are equal. This means that
12y - 9 = 15y - c
Given that c = 80, we would substitute it into the given equation. It becomes
12y - 9 = 15y - 80
- 9 + 80 = 15y - 12y
71 = 3y
3y = 71
y = 71/3
y = 23.67
Rounding to the nearest whole number,
y = 24