Here "head" is a metonym for the most important part. It is implied that Rezin is only a man, and therefore cannot stop Yahweh's plan. This can be stated explicitly. AT: "the king of Damascus is Rezin, who is only a man" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-explicit]])
Here "Ephraim" refers to all of the northern kingdom of Israel. This can be stated in active form. AT: "an army will destroy Ephraim, and there will no longer be a people of Israel" (See: [[rc://en/ta/man/translate/figs-activepassive]])
This means Pekah is the king of Samaria and all of Israel. AT: "the king of Samaria is Pekah, who is a weak man" (See: [[rc://en/ta/man/translate/figs-explicit]])
This can be stated in positive form. AT: "If you continue to believe in me, you will certainly remain safe" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])