A word ending with a sixth case-affix is not compounded with a word ending with tric or aka , when the force of the genitive case is that of an agent.,
The word 'kartari' qualifies the genitive-case. The affix तृच् is taught in sutra 3.1.133, and the affix अक is not a single affix; all affixes that have an element वृ are अक such as ण्वुवुल् or वुञ् ओर् वुन् ( sutra (VII. 1. 1.) ) Thus भवतः शायिका 'your honor's repose' भवत आशिका 'your eating' भवतोऽग्रगामिका 'your going in front.'
The affix तृच् is employed always in forming nouns of agency; hence there can be no example of a word in a genitive case having the force of an agent, governing another word also having the force of an agent. They therefore serves no purpose in this aphorism, but applies to the sutras that follow.
The genitive has the force of an agent under conditions mentioned in 2.3.65.
Why do we say 'when it denotes agent'? Observe इक्षुभक्षिकां मे धारयसि .,