Lily began a plan for this proof.

Since ∠1 and ∠2 are supplementary and ∠1 and ∠3 are supplementary, then m∠1+m∠2=180° and m∠1+m∠3=180°. If she can get m∠2 and m∠3 to both equal the same expression, she can use the transitive property of equality to set them equal to each other.

How can Lily get m∠2 and m∠3 to both equal the same expression?