1) m∠4 + m∠7 = 180° (given)
2) ∠4 is congruent to ∠6 (vertical angles)
3) m∠4 = m∠6 (definition of congruent angles)
4) m∠6 + m∠7 = 180° (substitution)
5) c is parallel to d (converse of same side interior angles theorem)
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