m∠x+m∠r+m∠y=180° (angles in a triangle add to 180°)
m∠r+m∠s=180° (angles that form a linear pair add to 180°)
m∠x+m∠r+m∠y=m∠r+m∠s (Transitive property of equality)
m∠x+m∠y=m∠s (subtraction property of equality)
m∠s=m∠x+m∠y (symmetric property of equality)
