Alternate exterior angles are formed by two parallel lines intersected by a transversal. These angles are found on the outside (exterior) of the parallel lines, i.e., they are not in between these two lines, and on opposite sides of the transversal. Alternate exterior angles are congruent. So, if there are two parallel lines and another line, the transversal, that crosses them, you can identify the outer angles that have the same measure.
In this problem we’re taking a fairly simple look at how can we apply alternate exterior angles. We have two parallel lines that are intersected by a transversal. If you notice x and 135 degrees are on opposite sides of this transversal and they’re on the exterior part of the two parallel lines which makes them alternate exterior angles which are always congruent, so x and 135 are the same number, so x is 135 degrees.