A rhetorical question with the answer included in the question. AT: "For if a man finds his enemy, he will not let him go safely." (See: [[rc://en/ta/man/translate/figs-rquestion]])
Where Saul had believed David was his enemy, he learns that even though David will be king, David will not take over the throne from Saul by force, but will wait for Yahweh's appointed time.
This can be stated in active form. Possible meanings are 1) David will cause the kingdom of Israel to prosper through his power as king. AT: "the kingdom of Israel will prosper as you rule the Israelite people" or 2) Yahweh will cause David to have complete control over the kingdom of Israel. AT: "you will have complete rule over the kingdom of Israel" (See: [[rc://en/ta/man/translate/figs-activepassive]])