We have to calculate the length of a side ( x ) of the rectangle. We know the perimeter of the rectangle: P = 24 ft. Formula for the perimeter of the rectangle is: P = 2 L + 2 W. The width W = 4 ft. So: 24 = 2 * x + 2 * 4; 24 = 2 * x + 8; 2 * x = 24 - 8; 2 * x = 16; x = 16 : 2; x = 8. Answer: The length of side of a rectangle is 8 feet.