It is given that rstu is a parallelogram, so ru∥st by the definition of parallelogram. therefore, ∠1≅∠2 by the alternate interior angles theorem, and m∠1=m∠2 by the . it is also given that ∠1 and ∠3 are complementary, so m∠1+m∠3=90° by the . by substitution, m∠2+ = 90°, and so ∠2 and are complementary by the definition of complementary.