It is always negative, because it keeps going down on the number line. Say it is negative 8 minus 5, it would be negative 13, because it adds to the negatives, so it would go deeper into the negatives.
If the width is 15, and the scale factor from width to length is 3:2, then you will have to divide 15 by 3 to get your original width. 15/3 = 5. 5 is your original width.
