We know that the four angles of a quadrilateral must sum up to 360º. So, we have:
Since X and Z are congruent, and W and Y are congruent, we have:
Thus, X and W are co-interior angles, which means segments XY and XZ are parallel.
And since Y and W are congruent, X and Y are also co-interior angles, which means segments XW and YZ are parallel.
Therefore, WXYZ is a parallelogram.