If you have a noun-noun compound that is possessed, then the possessive suffix replaces the -(s)i on the second noun.
So sırt çantası is "backpack", but "your backpack" is sırt çantan, which then forms the regular accusative by adding -ı.
And "my backpack" would be sırt çantam and "his backpack" would be ... sırt çantası.
So things such as "sırt çantası" are ambiguous between "backpack" and "his/her/its backpack". You could say onun sırt çantası to be explicit.