"Rashidi's pair of scissors"

Translation: Mkasi wa Rashidi

[MugodaSimo]

but why do we use "wa" here not "ya" pliz somebody

[AGreatUserName]

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.
Mikasi ya Rashidi. = Rashidi's pairs of scissors.

There's a table here
https://en.wiktionary.org/wiki/a#Inflection_6