In plain words
Therefore śāstra is your authority in determining what ought and ought not to be done. Knowing the karma that śāstric ordinance enjoins, you should perform it here.
Word for word
Therefore śāstra [is] your authority in settling what is to be done and not done; knowing the karma stated by the injunction of śāstra, you ought to do [it] here.
तस्माच्छास्त्रं प्रमाणं ते कार्याकार्यव्यवस्थितौज्ञात्वा शास्त्रविधानोक्तं कर्म कर्तुमिहार्हसि
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam· bhāṣya
तस्मात् शास्त्रं प्रमाणं ज्ञानसाधनं ते तव कार्याकार्यव्यवस्थितौ कर्तव्याकर्तव्यव्यवस्थायाम् । अतः ज्ञात्वा बुद्ध्वा शास्त्रविधानोक्तं विधिः विधानं शास्त्रमेव विधानं शास्त्रविधानम् ‘कुर्यात् , न कुर्यात्’ इत्येवंलक्षणम् , तेन उक्तं स्वकर्म यत् तत् कर्तुम् इह अर्हसि, इह इति कर्माधिकारभूमिप्रदर्शनार्थम् इति ॥ २४ ॥
In plain words
So scripture is your authority, your means of knowing, for deciding what should and should not be done. Which means: once you have truly understood what scripture declares -- and 'injunction' just means the rule, and scripture itself is that rule, so 'scriptural injunction' -- taking the form 'do this, do not do that' -- by that scripture, you should carry out the duty it prescribes as your own, right here. And 'here' does not just mean this place; it means the very domain in which you are qualified to act at all.
Close to the Sanskrit
Therefore, scripture is the authority, the means of knowledge, for you, in the determination of what is to be done and what is not to be done. Therefore, having known -- having understood -- what is declared by scriptural injunction -- 'vidhi' [means] 'vidhana,' scripture itself [is] 'vidhana,' [hence] 'scriptural injunction' -- having the character of '[one] should do [this], [one] should not do [that]' -- by that [scripture], the proper action enjoined, which you should perform here -- 'here' meaning for the purpose of indicating the sphere of eligibility for action.