The perimeter of a polygon is equal to the sum of all its sides. In a rectangle with dimensions length, x and width, y is calculated through the equation,
                   P = 2x + 2y
If width is y, then length is equal to 3y - 8. Substituting these to the equaiton,
                P = 2(3y - 8) + 2y
Simplifying,
             24 = 6y - 16 + 2y
              24 = 8y - 16
The value of y from the equation is 5. 
           x = 3y - 8 = 3(5) - 8 = 7
Answers: 
      x = 7 feet
      y = 5 feet