Who said this Pasuk to whom?
Rashbam: This Pasuk is a continuation of the father's answer to his son, the Tam. 1
How does this answer the question?
Seforno: This is a second answer to his question, inasmuch as Pharaoh (who is compared a donkey), 1 could have redeemed himself by sending Yisrael (who are compared to a lamb. 2 And because he refused, Hashem (Yisrael's Master), broke his neck (killed his firstborn).
Why does the Pasuk then conclude that the above is a reason for the Mitzvah of Pidyon ha'Ben?
Seforno: Because, in reality, the firstborn of Yisrael, who were unworthy of miracles, ought to have died together with their Egyptian counterparts. 1 And in order to rescue them, Hashem declared them holy - like Nezirim or perhaps even more, 2 requiring them to be redeemed.