Exegesis:

kai epērōtōn auton hoi ochloi ‘and the crowds asked him.’ The imperfect tense epērōtōn suggests that this question was asked repeatedly.

legontes ‘saying,’ has the force of a colon.

ti oun poiēsōmen ‘what then are we to do?’ oun implies that the question is a reaction to what John had said.

Translation:

What then shall we do? The question is not a rhetorical one, expressing despair, but a real one, asking for information how to bear fruits that are good in God’s eyes; hence such renderings as, ‘what is it God wants us to do?’ (Manobo), ‘what good shall we do?’ (Kekchi).

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.