What is the reason for the third step in this proof? given: a || b, and both lines

What is the reason for the third step in this proof? given: a || b, and both lines are cut by transversal t. prove: 2= 7
1. a || b given
2. 2 3 vertical angles theorem
3. 3 6
4. 2 6 transitive property of congruence
5. 6 7 vertical angles theorem
6. 2 7 transitive property of congruence
answers:
transitive property of equality
a) alternate interior angles theorem
b) corresponding angles theorem
c) same-side interior angles theorem
d) vertical angles theorem