Let's call the width w. That makes the length equal to 2w.
The perimeter to two times the length and two times the width, or
Perimeter = 2(2w) + 2(w)
That equals 4w + 2w or 6 w.
So if the perimeter is 14, we can say that 14 = 6w.
Divide by 6 to get w = 14/6 or 7/3.
The width is 7/3 feet or 2 and 1/3 feet, and the length is twice that or 14/3 or 4 and 2/3.