check the picture below.
notice the tickmarks, is a given, that those sides are congruent.
now, look at the side at the bottom, the BC side, is really shared by both triangles, and therefore, the same length for both. Meaning we might as well put a tickmark on that one too.
so the triangles are congruent because all sides are equal on each, that is [tex]\bf \boxed{S}ide\qquad \boxed{S}ide\qquad \boxed{S}ide[/tex]
