On this problem we have two triangles FLW and YLW. We know that the sides FL and LY are congruent and the side LW is shared between the two triangles. Therefore we know that two sides are congruent, we don't know however if the angles F and Y are congruents, there is no indication of that, so we can't prove that it is congruent, because we need to know if the angles are congruents.