2.
So far, so good. Now you cite the transitive property of congruence, saying objects congruent to congruent objects are congruent.
(IMO, another step might be needed. You have 2 ≅ 1 ≅ 7 ≅ 5, but you may have to show 1 ≅ 5 or 2 ≅ 7 before you can jump to 2 ≅ 5. The transitive property is usually written in terms of only one intermediate: a ≅ b ≅ c ⇒ a ≅ c.)