Answer
In these two triangles, two sides on each of the triangles are marked congruent. The third side, $\overline{CE}$, is shared by both triangles, so it is also congruent. Therefore, we can prove that these triangles are congruent using the SSS theorem, which states that if the three sides of one triangle are congruent to three sides of another triangle, then those two triangles are congruent. We can say that $\triangle BCE$ is congruent to $\triangle DCE$. Congruent triangles have congruent angles, so $\angle B ≅ \angle D$.
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