When comparing the angles of a triangle, they are written with the matching angles in the same order. (sorry for bad explanation here)
You have ∆ QRS as congruent to ∆ XYZ. This means angle Q is congruent to angle X, angle R is congruent to angle Y, and angle S is congruent to angle Z. <u>As you can see in ∆ XYZ, angle Z is 50°. Therefore, angle S and angle Z are both 50°</u> (but only angle S is an option)