Triangles FGH and ABC are congruent because of the: AAS congruence theorem.
The AAS congruence theorem states that when two angles and one non-included side in one triangle are congruent to corresponding two angles and one non-included side in another triangle, then both triangles are congruent.
In the proof given, it is established that both triangles have two corresponding congruent angles, and also, BC ≅ GH which are non-included sides.
Therefore, both triangles are congruent because of the AAS congruence theorem.
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