"Have a good afternoon, Rehema"
Translation:Mchana mwema Rehema
It depends on the noun class of the thing it refers to. Mchana is in the m-mi class (although it generally has no plural because it's uncountable), so it's mwema. Another word like jioni for example, which belongs to the N-N class, will be described as njema.
There's a table of all the forms here:
Don't get overwhelmed by the number of forms yet. Mostly the prefixes are similar to the prefixes on nouns and this course takes you through the classes one at a time.
Because adjectives in Swahili have a different prefix depending on the class of the noun they refer to/correlate with. "mchana" is class 3 (plural in class 4 "michana") and therefore the adjective "-ema" will always become "mwema". "Njema" is used with nouns of the n-classes (9 and 10).