वाचंयमपुरंदरौ च

Operational rule vidhi

वाचंयम and पुरंदर are irregularly formed.

Word by word (padaccheda)

Substitute / term वाचंयम- पुरंदरौ prathamā · dvivacanaIndeclinable च avyaya

Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Full reading

वाचंयम-पुरंदरौ च

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

Commentary

वाचंयम and पुरंदर are irregularly formed.
Thus वाचंयमः आस्ते 3.2.40 खच् affix)). पुरं दारयति = पुरन्दरः 3.2.42. The shortening of याम and दार takes place by 6.4.94.

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