What is the value of c ?

Given: in quadrilateral wxyz , and bisect each other at point a. prove: quadrilateral wxyz is a parallelogram. statement reason 1. and bisect each other. given 2. wa = ya xa = za definition of bisection 3. m∠yaz = m∠xaw vertical angles theorem 4. sas criterion for congruence 5. ∠ ywx ≅ ∠wyz corresponding angles of congruent triangles are congruent. 6. || converse of alternate interior angles theorem 7. m∠xay = m∠zaw vertical angles theorem 8. ∆xay ≅ ∆zaw sas criterion for congruence 9. ∠xzw ≅ ∠zxy corresponding angles of congruent triangles are congruent. 10. || converse of alternate interior angles theorem 11. quadrilateral wxyz is a parallelogram. definition of parallelogram what is the missing statement in this proof? what is the missing statement in this proof?