I assume the 2 vertical lines are parallel.
then the 2 angles 5x and 4x are around the intersection point of both lines (since parallel lines simply mimic the behavior of each other, the angles must be the same).
that means 4x is also the angle under the indicated 5x angle, and 5x is also the angle under the indicated 4x angle.
and that simply means that 5x and 4x supplement each other to 180° (as all angles around a single point on one side of a line represent a half-circle and therefore 180°).
5x + 4x = 180
9x = 180
x = 20