तस्माच्छास्त्रं प्रमाणं ते कार्याकार्यव्यवस्थितौ। ज्ञात्वा शास्त्रविधानोक्तं कर्म कर्तुमिहार्हसि ॥

tasmāch chhāstraṁ pramāṇaṁ te kāryākārya-vyavasthitau jñātvā śhāstra-vidhānoktaṁ karma kartum ihārhasi

Therefore let the Scriptures be thy authority, in determining what ought to be done, or what ought not to be done. Knowing what hath been declared by the ordinances of the Scriptures, thou oughtest to work in this world.

In plain words: So, Krishna tells Arjuna, let scripture be your guide for what to do and what not to do. Learn what it teaches, and then act in the world.