Explanation:
when a line intersects 2 parallel lines, the intersection angles must be equal with each of the parallel lines. otherwise they are not parallel.
and the sum of all angles around a single point on one side of a line is always 180°.
in the case of such intersection angles it simply means that both angles at the intersection on one side of the line are supplementary (together they have 180°).
so,
180 = 75 + 15x
15x = 105
x = 105/15 = 7