"How is your afternoon, Rehema?"
Translation:Habari za mchana Rehema?
Ya is for class 9 and za for class 10. Class 9 is singular and class 10 is plural, so, for example:
nyumba ya watu = the house of the people (nyumba is class 9 here, "house")
nyumba za watu = the houses of the people (nyumba is class 10 here, "houses")
It might be confusing with habari because "news" is uncountable in English, but you can think of habari as essentially meaning "piece of news", "report", "bit of information". So in theory, you could say habari ya ... if you only want one piece of information and habari za ... if you want more.
However, in practical use, no one wants any information other than nzuri or something. It's simply more common to use za after this kind of greeting with habari. Habari
yako is the only exception I can think of, where the "y" form is more common.