Exercise 9 Page 630

Let's draw $△ABC$ and $△DEF.$

Next, we will mark the two pairs of congruent angles, $\angle A\cong \angle D$ and $\angle C\cong \angle F,$ and the congruent sides, $\overline{AC}\cong \overline{DF},$ in our diagram.

Since we know two pairs of angles and the pair of included sides are congruent, we can prove that the triangles are congruent by the ASA Congruence Theorem.