[
  "Let width = W. Length = 2W + 5. Perimeter = 2(L+W) = 70.",
  "Substitute: 2(2W+5+W) = 2(3W+5) = 6W+10 = 70.",
  "6W = 60, W = 10. Length = 2(10)+5 = 25 cm."
]