The XC segment is a common side for triangle ABC and triangle XYZ because they are overlaping and the segments AX≡XZ then AC≡XZ
According to the SAS criteria (side-angle-side), two sides (AB≡YZ and AC≡XZ) and the included angle (∠A≡∠Z) are respectively equal, then the triangles ABC and XYZ are congruents, so their angles are equal, for those reasons ∠B≡∠Y
