Given an isosceles triangle $\triangle{BAC}$ as the one in the figure below and the reflection in line $l_{BC}$ that transforms the triangle into $\triangle{B'A'C'}$. How can I prove that $s_{AB} \parallel s_{C'A'}$? I know both sides are congruent and reflections keep angles and measures.This is the first step of a problem solution. However, it doesn't mention a theorem or an axiom so it has to be extremely easy, but I'm failing to see it.
Note: ${B=B'}$ and ${C = C'}$. I'm still learning how to use geogebra and this is also my first geometry course. :)