In chapter 14 what does aunt Alexandra want Atticus to do about calpurnia? Why do you think she wants that?

Aunt Alexandra finds out that Scout and Jem went to the black church with Calpurnia and tries to forbid Scout from visiting Calpurnia's home. Moreover, Aunt Alexandra tries to make Atticus fire Calpurnia, but he refuses on the grounds that she's done an excellent job of running the house and raising the children, and the children love her.