Our objective is to prove that angle 3 is congruent to angle 6, and that angle 4 is congruent to angle 5. Step 2 has shown that angle 3 is congruent to angle 7, so we just need to prove that angle 7 is congruent to angle 6. Similarly, Step 2 shows angle 4 as congruent to angle 8, so we need to prove that angle 8 is congruent to angle 5. Both of these are accomplished using the definition of vertical angles, which is the fourth of the given choices.
