what id the routine of rabbi?

but didnt Sarah know that Hashem promised her a son from Avraham.One of the Malachim that came to visit Avraham told him he would have a son with Sarah.So then why did Avraham marry Hagar?

this is a test

why did Avrham have to marry Hagar in the firt place?Why not just have one wife Sarah? If Hagar was a righteous person then how is it that the Arab nation came out of her, the nation that is causing the Jews so much trouble? Thanks for this great survace

thank you.