"Rashidi's pair of scissors"
Translation:Mkasi wa Rashidi
The choice between the different forms of -a depends on the noun class of the word it refers back to. Mkasi is class 3, the singular in the M-MI class, so it takes wa.
If it were plural, it would be ya.
ya Rashidi. = Rashidi's pairs of scissors.
There's a table here https://en.wiktionary.org/wiki/a#Inflection_6