This is a request. To carry out what one has said is an idiom meaning to do what one has said. Alternate translation: "please do what you have promised" (See: [[rc://en/ta/man/translate/figs-idiom]])
God speaking in terms of David having a man implies that the man would be a descendant of David. This can be stated positively. Alternate translation: "you will always have a descendant" (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/ta/man/translate/figs-doublenegatives]])
# a man in my sight to sit on the throne of Israel
The phrase "in my sight" here implies that God would choose the man and the man would want to obey God. (See: [[rc://en/ta/man/translate/figs-explicit]])
The throne is a metonym for the activity of the one who sits on the throne. Alternate translation: "to rule over Israel" (See: [[rc://en/ta/man/translate/figs-metonymy]])
The way a person behaves is spoken of as if that person were walking on a path. Alternate translation: "to obey my law, as you have obeyed me" or "to be faithful to my law as you have been faithful to me" (See: [[rc://en/ta/man/translate/figs-metaphor]])