If those lines are to be parallel, those two marked angles (one acute, one obtuse) will be supplementary (add to 180°).

Equation:

5x + 12 + 6x - 8 = 180

11x + 4 = 180

11x = 176

x = 176/11

x = 16

We know they add together to 180°

5x +12 + 6x -8 = 180