Exercise 1 Page 651

We already know the following is true. We also have to show that the included angles, $\angle ACB$ and $\angle DCE$ are congruent as well.

Angles

If we look at the diagram, we see that $\angle ACB$ and $\angle ECD$ are vertical angles. According to the Vertical Angles Congruence Theorem, vertical angles are congruent.

Conclusion

Since we now know that two sides and the included angle of $△ABC$ are congruent to two sides and the included angle of $△EDC$, we can claim by the SAS Congruence Theorem that the triangles are congruent.