The relationship between <7 and <8 can be said to be vertically opposite angles. They are equal to one another and formed by the intersection of two straight lines. i.e. the intersection of lines C and D.
The special angles created by the intersection of lines C and D can be used to solve for y because they all are inter woven and connected to one another either supplemtarily or by being vertically opposite angles.
5y - 29 = 3y + 19( vertically opposite angles)
5y - 3y = 19 + 29
2y = 48
y = 48/2
= 24.
<7 = <8
<7 + 5y - 29 = 180°( angles on a straight line)
<7 = 180° + 29 - (5 × 24)
= 209 - 120
<7 = 89°
<8 = 89° (vertically opposite angles)