the first thing you might realize from your traditional geometry is that this a triangle that has two angles that are the same and if the two angles are the same the two sides here are going to be the same aswell so we know from our geometry that this side is equal to that side. some times it's easier to visualize if you have a triangle like this and these two base angles are congruent then you know that these two sides are congruent this is one of the fundamental things that we learned in geometry, this is just reoriented in a different way.