सहेः पृतनर्ताभ्यां च

Operational rule vidhi

ष् is substituted for the स् of सह after पृतना and ऋत.

Word by word (padaccheda)

Target सहेः ṣaṣṭhī · ekavacanaLeft context पृतना- ऋताभ्याम् pañcamī · dvivacanaIndeclinable च avyaya

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Full reading

सहेः पृतना-ऋताभ्याम् च

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

ष् is substituted for the स् of सह after पृतना and ऋत।
Thus पृतनाषाहम्, ऋताषाहम् ॥ Some divide this sutra into two; as (1) सहेः; (2) पृतनार्त्ताभ्यां च ॥ So that the rule may apply to ऋतीषहः also. The word ऋति is lengthened by 6.3.116, in sanhita : and the ष change takes place in sanhita only. Otherwise the form is ऋतिसहम् ॥ The word च 'and' in the sutra includes words other than those not mentioned in it. The word ऋतीषहं would be valid by this also.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.