Answer:
5. Substitution
6. Substitution
Explanation:
The properties of congruence generally match those of equality;
A ≅ A . . . . reflexive property
A ≅ B, B ≅ C ⇒ A ≅ C . . . . transitive property
A ≅ B, A ≅ C ⇒ B ≅ C . . . . substitution
__
In this proof, the conclusions (statements 5 and 6) are based on substitution. In each case, the elements of the congruence have previously been shown to be congruent to the same thing. This is the setup for making use of the substitution property of congruence.