Answer: the answer is this
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.
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.
Using substitution, we have m∠ZAB + m∠CAB = m∠CAB + m∠ACB + m∠CBA.
Therefore, we conclude m∠ZAB = m∠ACB + m∠CBA using the
✔ subtraction property.
Explanation: I got it correct this is show down below.