Rashi on Numbers 22:6/רש"י על במדבר כ״ב:ו׳

1נכה בו (this may mean, “we may smite them”) I and my people may smite them. — Another explanation is that this is a Mishnaic Hebrew expression as in (Bava Metzia 105b) “he deducts (מנכה) for him something from the price”, so that it means: [perhaps I may be able] to reduce them a little in numbers (Midrash Tanchuma, Balak 4).

2כי ידעתי וגו׳ FOR I KNOW etc., through the war of Sihon against Moab, because you assisted him to smite Moab (Midrash Tanchuma, Balak 4; cf. Rashi on 21:27).