Bg 3.20

karmaaiva hi sasiddhim
āsthitā janakādaya
loka-sa
graham evāpi
sampa
śyan kartum arhasi

Word for word: 

karmaṇā — by work; eva — even; hi — certainly; sasiddhim — in perfection; āsthitāḥ — situated; janaka-ādaya — Janaka and other kings; loka-sagraham — the people in general; eva api — also; sampaśyan — considering; kartum — to act; arhasi — you deserve.

Translation: 

Kings such as Janaka attained perfection solely by performance of prescribed duties. Therefore, just for the sake of educating the people in general, you should perform your work.

karmaaiva hi sasiddhim, by performing Karma itself sasiddhim, perfection is attained by whom? āsthitā janakādaya, by Maharaj Janaka and other persons like him, loka-sagraham evāpi, therefore for the purpose of loka-sagraha, sampaśyan kartum arhasi, sampaśyan means see systematically, consider, loka-sagra means setting examples of others sagra actually means accumulation or maintenance so maintain the lokas, how are the lokas maintained? By proper example. loka-sagraham evāpi

sampaśyan kartum arhasi. So considering that you should act properly, so He is saying in the past also there were many people who were liberated by doing there prescribed duty so don’t think that prescribed duty cannot bring liberation, by doing one’s prescribed duty also one can move forward, and eventually becomes liberated.

So in this section Lord Krishna told Arjuna it is not external duites which causes bondage but one’s internal mentality, if one is detached then one is not bound but by being detached if one does one’s duty then one is able to set a good example for other’s also, so for loka-sagraha, please do your duties and in the next verse Krishna will talk about Himself also and that will be the first verse where Krishna has talked about His supreme position explicitly, He has hinted in 2.61 but now He will talk more directly in 3.22 and  3.23.