We start with triangle ABC and see that angle ZAB is an exterior angle created by the extension of side AC. Angles ZAB and CAB are a linear pair by definition.
Answer
We know that m∠ZAB + m∠CAB = 180° by the .:
angle addition postulate
We also know m∠CAB + m∠ACB + m∠CBA = 180° because .:
of the triangle angle sum theorem
Therefore, we conclude m∠ZAB = m∠ACB + m∠CBA using the .: