It's a flub of her trying to say "I'll do it." In the raw she says "す- するう... っ" (Su- suruu...). "I do" said like they do during wedding ceremonies in Japanese is "はい誓います" (Hai. Chikaimasu).Not sure what this "I do it" was about, she clearly says "I do" as they do in church when getting married, but thanks for the TL