In order to determine what is the value of x, it is necessary that you consider that angle next to the angle with 124° is supplementary to this angle (124°).
It means that they add up 180°.
Then, such angle sext to the angle with 124° is:
180 - 124 = 56
The angle next to angle with 124° has a measure of 56°.
Next, take into account that two parallel lines intersect the secant line. It means that both angle of 54° and angle with a mesaure of x are equal.
Hence, the value of x is x = 54°