Answer: SSS
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Step-by-step explanation:
The sides shown by the single tickmark are the same length. That's one "S" of "SSS".
The sides shown with double tickmarks are the same length. This is another "S" of "SSS".
Lastly, the third unmarked sides of each triangle overlap together perfectly. We consider this a shared side. They are the same length due to the reflexive property. This is the third "S" of "SSS".
The order of the "S" terms mentioned above doesn't matter. All that matters is that we have three pairs of congruent sides. This is enough to use the SSS congruence theorem to prove the two triangles are congruent.
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Extra info:
We don't have any information about the angles, so we cannot use ASA, SAS, or AAS.
We can't use HL because that only applies to right triangles.