Answer
1. WXYZ is a parallelogram--given
2. Opposite angles in parallelograms are congruent.
3. Thus, X is congruent to Z, and W is congruent to Y.
4. Diagonals bisect the angles of parallelograms.
5. Thus, the set of two angles at the base of each triangle are the same.
6. By the vertical angles theorem, angle ZMY is congruent to angle WMX.
7. By AAA, triangle WMX is congruent to triangle YMZ.
