After folding, you get an open box. Its base is the square with the side length of (x-2*4) = x-8 cm, where x is the side of the original cardboard. The height of the box is 4 cm. Hence, 4*(x-8)^2 = 144 is the volume equation. It gives (x-8)^2 == 36. Hence, x-8 = = 6. Then x = 6+8 = 14. Answer. The side length of the original square piece of cardboard is 14 cm.