SB 1.7.44

sarahasyo dhanur-vedaḥ
 savisargopasaṁyamaḥ
astra-grāmaś ca bhavatā
 śikṣito yad-anugrahāt
Synonyms: 
sa-rahasyaḥ — confidential; dhanuḥ-vedaḥ — knowledge in the art of manipulating bows and arrows; sa-visarga — releasing; upasaṁyamaḥ — controlling; astra — weapons; grāmaḥ — all kinds of; ca — and; bhavatā — by yourself; śikṣitaḥ — learned; yat — by whose; anugrahāt — mercy of.
Translation: 
It was by Droṇācārya’s mercy that you learned the military art of throwing arrows and the confidential art of controlling weapons.
Purport: 

Dhanur-veda, or military science, was taught by Droṇācārya with all its confidential secrets of throwing and controlling by Vedic hymns. Gross military science is dependent on material weapons, but finer than that is the art of throwing the arrows saturated with Vedic hymns, which act more effectively than gross material weapons like machine guns or atomic bombs. The control is by Vedic mantras, or the transcendental science of sound. It is said in the Rāmāyaṇa that Mahārāja Daśaratha, the father of Lord Śrī Rāma, used to control arrows by sound only. He could pierce his target with his arrow by only hearing the sound, without seeing the object. So this is a finer military science than that of the gross material military weapons used nowadays. Arjuna was taught all this, and therefore Draupadī wished that Arjuna feel obliged to Ācārya Droṇa for all these benefits. And in the absence of Droṇācārya, his son was his representative. That was the opinion of the good lady Draupadī. It may be argued why Droṇācārya, a rigid brāhmaṇa, should be a teacher in military science. But the reply is that a brāhmaṇa should become a teacher, regardless of what his department of knowledge is. A learned brāhmaṇa should become a teacher, a priest and a recipient of charity. A bona fide brāhmaṇa is authorized to accept such professions.