The affixes ṇa and yat come respectively after the words yathā-kathā-ca and hasta being in the Instrumental -3rd case in construction having the sense of what is 'given or is done in this way'.,
The words दीयते and कार्य apply to both; the rule of यथासंख्यः (1. 3. 10) not applying to it. The word यथाकथाच is an Indeclinable, meaning contempt. Of course this word cannot take any case-affix, it therefore only gives the sense of third case. As यथाकथाच दीयते कार्यं वा = याथाकथाचम्, हस्तेन दीयते कार्यं वा = हस्त्यम् ॥,