PQRS is a parallelogram Given SR=PQ property of parallelogram m∠S=m∠Q property of parallelogram SP=QR property of parallelogram XP=RY given SP-XP=QR-RY substitution SX=QY segment subtraction ΔSRX is conggruent to ΔQPY SAS theorem (side-angle-side) XR=YP CPCTC (corresponding parts of congruent triangles are congruent)