@jflores26 There really is not a way to do this in Canvas.  Since you are open to the idea of doing it in Excel, that can be done very easily.  This is ultimately the formula that you would use: =(A1*0.3)+(B1*0.7) 

You can replace any of these values based on where your scores are but using your example, the score for the first submission is in cell A1 and the ".3" is weighting it 30%.  Then the score of the revised version is in cell B1 which is being weighted at 70% (.7).  I uploaded an excel sheet you can use with everything in place.  You can use the top right corner in green to enter your weights (make sure they add to 100) and the weighted score will update accordingly so you do not have to edit any of the formulas.

Hope this helps!

-Nick