Now know I. The employment of the first person singular marks a change in the speaker, and is best explained by supposing that either the high priest or the king himself takes the word. The offering of the solemn prayer (Psalms 20:1-5) and of the sacrifices (see the comment on Psalms 20:3) has been followed by a full conviction that the prayer is granted, and the triumph of David assured.
What was previously hoped for is "now known." That the Lord saveth (or, hath saved) his anointed (comp. Psalms 18:50). He will hear him from his holy heaven; literally, from the heaven of his holiness.
With the saving strength of his right hand. God will hear him, i.e; and, having heard him, will help and defend him "with the saving strength of his right hand."