We are given that m∠1 = m∠2. We are to prove that line l is parallel to line m. So, let's make a proof. Your first statement should always be the given statement. You are given the statements. All we have to do is find the reasons for those statements.
m∠1 = m∠2 → Given
m∠1 = m∠3 → Vertical angles are equal
m∠2 = m∠3 → Substitution
l ║m → If corresponding angles are equal, then lines are parallel
