Explanation:
it is simple, really.
the interception angles of parallel lines with another, intersecting line are the same for every parallel line.
so, the angle at the intersection point are the same for the lines m and n.
that means the angle on the right side of (x - 3) is (8x + 3).
and the angle on the left side of (8x + 3) is (x - 3).
now, the sum of all angles around a single point on one side of a line is always 180°.
therefore,
(x - 3) + (8x + 3) = 180
x - 3 + 8x + 3 = 180
9x = 180
x = 180/9 = 20