this triangle is separated into two right triangles
if AB=BD(given) so the hypotenuses are congruent
one of the sides is same between the right triangles (BC)
so according to HL theorem, if the hypotenuses and one of the sides (except hypotenuse) between two or more right triangles are congruent to each other
the triangles are congruent
hope you understand it
good luck
