Here "throne" refers to the power to rule as king. The abstract noun "faithfulness" can be stated as "faithful." This can be stated in active form. Alternate translation: "Yahweh will be faithful to the covenant and he will appoint a king" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-abstractnouns]] and [[rc://en/ta/man/translate/figs-activepassive]])
Here "David's tent" represents David's family, including his descendants. Sitting on the throne represents ruling. Alternate translation: "a descendant of David will rule faithfully" (See: [[rc://en/ta/man/translate/figs-metonymy]])