I am given that p || q. ∠1 and ∠4 are supplementary angles because they form a linear pair, so m∠1 + m∠4 = 180°. ∠4 and ∠8 are also supplementary because of the Same-Side Interior Angles Postulate, so m∠4 + m∠8 = 180°. You can substitute m∠4 + m∠8 for 180° in the first equation above. The result is m∠1 + m∠4 = m∠4 + m∠8. After subtracting m∠4 from each side, I see that m∠1 = m∠8.