Question

which could be the missing reason in step 3?

Explanation:

Step1: Recall the property of alternate interior angles

When two parallel lines are cut by a transversal, alternate interior angles are congruent.
In this case, \(y\parallel z\) and the lines \(AC\) and \(AB\) act as transversals.
For \(\angle1\) and \(\angle5\), line \(AC\) is the transversal cutting the parallel lines \(y\) and \(z\).
For \(\angle3\) and \(\angle6\), line \(AB\) is the transversal cutting the parallel lines \(y\) and \(z\).

Answer:

Alternate interior angles theorem.