Remember from your geometry that when a line intersects with 2 parallel lines, the opposite exterior angles are congruent. The angles line l makes with line n are the same as the angles line l makes with line m, and angles opposite each other are congruent. Therefore, angle y is congruent to the angle across from it, which is in turn congruent to x because it is also the lower left angle made by line l. The answer is C.