Exegesis:

kai ouk ischusan antapokrithēnai pros tauta ‘and they were not able to make a reply to this.’ For ischuō cf. on 6.48.

antapokrinomai ‘to make a reply,’ here of making a reply to a question in an argument.

Translation:

Could not reply to this, or, ‘could not answer (or, meet) this question,’ ‘had nothing to say against this, or, to refute this (i.e. the criticism implied in Jesus’ question)’; or again, emphasizing their inability, ‘could not answer, not a single word (lit. one-movement-of-the-lips not)’ (Tae’).

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.