Answer: One vase should be placed 3 feet from the end of the mantel and the other vase should be placed 9 feet from the same end.
Step-by-step explanation:
You know that lenght of the mantel above their fireplace is 12 feet.
They want to place one vase of the distance from one end of the mantel. This distance in feet is:
They want to place the other vase of the distance from the same end of the mantel. Therefore, this distance in feet is:
Therefore, one vase should be placed 3 feet from the end of the mantel and the other vase should be placed 9 feet from the same end.