If you're dividing -15 by w then the answer would be -15/w. This is because the value of w has not been determined yet and a number cannot be divided by a variable.
If you're dividing w by -15, the same situation would occur due to the fact that a variable cannot be divided by a number. The answer would be w/-15.
A variable is merely a "placeholder", for lack of better terms, for a number(s) that have not been identified yet.
I hope I cleared this up!