Explanation:
it seems the questions 8., 9. and 10. demonstrate exactly what is the asked via variables in 11., 12. and 13.
you only need to apply the principles of equal angles of crossing lines again.
11.
to be parallel
50 + (x + 30) = 180
x + 80 = 180
x = 100.
12.
135 = 2y + 15
120 = 2y
y = 60
13.
70 = 4x - 10
4x = 80
x = 20
14.
yes, because
142 + 38 = 180°
15.
yes, because
112 + 68 = 180°
16.
no, because
146 + 44 = 190° <> 180°
17.
to be parallel, angle 1 = angle 2
88 = 2x
x = 44
18.
to be parallel, angle 1 = angle 3
88 = 2x + 10
2x = 78
x = 39