1. We are given that WX is parallel to YZ. 2. If WX is parallel to YZ, angle Z and W are congruent, and angles Y and Z are congruent by the corresponding angles theorem. 3. Since Z is supplementary to Y, X and W are also supplementary. 4. Thus, ZW and XY are parallel. 5. Since both sets of sides are parallel, it is a parallelogram.