Voltage drop polarity
The negative sign in the answer indicates that the polarity of the voltage is opposite to that marked on the diagram. 

You mention that twice in your working, but forgot to include it in the result.


  Vs will have a positive terminal on the bottom of the resistor and negative terminal on top of the resistor.


After completing the calculations an noting the polarity is opposite that shown, you should invert the number you got to get the final result