Luke said that when Jesus heard this (ὁ δὲ Ἰησοῦς ἀκούσας), he replied to the synagogue leader (ἀπεκρίθη αὐτῷ) that he should not fear (Μὴ φοβοῦ), but only believe (μόνον πίστευσον), because she would be saved (καὶ σωθήσεται). Mark chapter 5:36, was similar to Luke here, almost word for word. However, Matthew did not have this conversation, since the child was dead from the beginning. Mark said that Jesus overheard what they were talking about. He then turned to this synagogue leader and told him not to fear, but only believe. Belief was a key element in many of these healing cases with Jesus. Would you believe in Jesus at a time of death?
In case there was any doubt about what covenant or agreement Zechariah was talking about, he made it clear, via this canticle in Luke, that this was the covenant or agreement with Abraham to have him and his many descendants be prosperous as in Genesis, chapter 22:16-18. Zechariah said that God had remembered or given them (τοῦ δοῦναι ἡμῖν) the oath (ὅρκον) that he swore (ὃν ὤμοσεν) to Abraham (πρὸς Ἀβραὰμ), their ancestor or father (τὸν πατέρα ἡμῶν). The covenant was coming about right before their very eyes. Notice it was not the Mosaic, but the older Abrahamic covenant that Luke emphasized.
Only Matthew has this question for his disciples. Jesus wanted to be sure that his disciples understood what he was talking about. Thus, he asked them (λέγουσιν αὐτῷ) if they understood everything that he was talking about (Συνήκατε ταῦτα πάντα). They responded that they did with a positive yes (Ναί). Obviously, some may have understood better than others. However, they all understood something.