Bg 6.39

etan me samsayam Krishna
chettum arhasy asesatah
tvad-anyah samsayasyasya
chetta na hy upapadyate

Word for word: 
etat — this is; me — my; samsayam — doubt; Krishna — O Krishna; chettum — to dispel; arhasi — You are requested; asesatah — completely; tvat — than You; anyah — other; samsayasya — of the doubt; asya — this; chetta — remover; na — never; hi — certainly; upapadyate — is to be found.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This is my doubt, O Krishna, and I ask You to dispel it completely. But for You, no one is to be found who can destroy this doubt.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Krishna is the perfect knower of past, present and future. In the beginning of the Bhagavad-gita, the Lord said that all living entities existed individually in the past, they exist now in the present, and they continue to retain individual identity in the future, even after liberation from the material entanglement. So He has already cleared up the question of the future of the individual living entity. Now, Arjuna wants to know of the future of the unsuccessful transcendentalist. No one is equal to or above Krishna, and certainly the so-called great sages and philosophers who are at the mercy of material nature cannot equal Him. Therefore the verdict of Krishna is the final and complete answer to all doubts, because He knows past, present and future perfectly – but no one knows Him. Krishna and Krishna conscious devotees alone can know what is what.