"Jioni njema Rehema"
Translation:Have a good evening Rehema
If you go to the lesson on adjectives and click on the tips & notes (light bulb), there is a lot of information on adjective agreement.
The form mwema is used with the M/WA noun class, e.g.:
"mwanafunzi mwema" = the good/kind/nice student .
Nouns in class N/N (including "jioni") take a rather complicated variety of prefixes on the adjective, usually m-, n- or nothing. But point 4 says that the adjective -ema is an exception, as the prefix becomes nj- with a class N/N noun.
So: "Jioni njema" = Good evening, "soksi njema" = nice socks.