Recall, that we will be working with a square ABDE.
We want to find out what type of triangle ABC is.
To begin, choose a point F inside the triangle such that triangle BFD is congruent to triangle DCE.
Notice that BF is perpendicular to CD of the equilateral triangle CDF.
By the previous statement, BC = BD = AB.
Therefore, we can conclude
that triangle ABC is equilateral.