Yahweh is speaking to the mountains. This can be stated explicitly. Alternate translation: "So, mountains of Israel, I will multiply upon you" (See: [[rc://en/ta/man/translate/figs-explicit]])
This can be stated in active form. Alternate translation: "Then people will live in the cities and will rebuild the ruins" (See: [[rc://en/ta/man/translate/figs-activepassive]])