First Answer: Transitive property of Equality
Explanation: If x = y and z = y, then x = z. This is the basic template. In this case, both left side expressions of the problem are equal to 180, so they must be equal
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Second Answer: Alternate Interior Angles Theorem
Explanation: These angles are on the inside of the parallel line train tracks (hence the "interior") and they are on opposite sides of the transversal line (hence the "alternate"). Alternate interior angles are congruent if the lines are parallel, which they are in this case.
