Audible dua after salaah

Q: In some coutries, ulama say that zikr (like Lailaha illahu wahdehu la shareeka lehu... zal jadda minkal jadd) which is done after prayers with loud voice is sunnah. We have never heard our ulama or Imamas in Pakistan doing zikr after prayers with audible voice. Can you please guide what is Sunnah?

Black magic

Q: I was an extraordinarily bright student with achievements of state level and 4 gold medals that I received from Chief of the Air staff but all of a sudden I began to feel frightened drowsy and having intrusive thoughts. Its been 7 years now and my sufferrings are not ending. I think a relative who I suppose was jealous of my achievements has casted a spell of some sort of magic on me. He has no child even after 10 years of marriage. What is the truth of black magic in Islam and how can we break its spell?

Marrying one's ex-wife's daughter

Q: Is it possible to marry an ex-wife's daughter? I know a man that married a woman 13 years older and she never told him she has a daughter almost his age. They are not together any longer and he wants to propose to the daughter. Is this permissible considering he didn't know this? He didn't have intercourse with her since finding out as he was very hurt and shocked.