We already know 2 of the S's since it's given that AB is congruent to CD and BC is congruent to AD. To get the last "S" of "SSS," we know that BD and DB are congruent by the reflexive property. Knowing that these triangles are congruent using SSS, you can go on to prove what it's asking you to prove, but for this question, I'm pretty sure you just need to know to use SSS.
