The square frame poster has an area of 9 square feet. Therefore each side is 3 feet long. For the poster to lie flat, the cube shaped box must be at least 3 feet long and 3 feet wide. If this were the case, then the cube shaped box would have a volume of 3 x 3 x 3 = 27 cubic feet. Since the volume of the box is more than 27 cubic feet, the box is large enough for the poster to lie flat.