Explanation:
3. ∠2 and ∠3 are a linear pair → definition of a linear pair
4. ∠3 and ∠4 are a linear pair → definition of a linear pair
5. m∠2 +m∠3 = 180° → definition of a linear pair (or angle addition postulate)
6. m∠3 +m∠4 = 180° → definition of a linear pair (or angle addition postulate)
7. m∠2 +m∠3 = m∠3 +m∠4 → substitution property
8. m∠2 = m∠4 → subtraction property
9. ∠2 ≅ ∠4 → definition of congruent angles
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A pair of angles is a "linear pair" if they are adjacent and supplementary. ∠2 and ∠3 are adjacent and the non-common legs form a straight line. It isn't clear what your definition of linear pair is and what you need to do to claim that the angles sum to 180°. Above, we have assumed a definition of "linear pair" that includes the facts that → they sum to 180°; → non-adjacent sides form a straight line.
