In order to determine the length of the sides, factorize the given expression for the area of the table top:
9x² - 48x + 64
take into account that the previous expression is a perfect square, in fact, you have:
9x² - 48x + 64 = (3x - 8)² = (3x - 8)(3x - 8)
Hence, the length of each side is 3x - 8