On that diagram, the unmarked angle above the (6x-8) is equal to (5x+12). This is because the unmarked angle and the angle marked with (5x+12) are corresponding angles. Corresponding angles must be congruent if you want line m to be parallel to line n.
From here we add up (5x+12) and (6x-8) and set that sum equal to 180. Solve for x
