Issues with SumX Function -- Table Totals Incorrect

Hello Greg,
Thank you for your quick response. I can't believe I watched one of your videos earlier and you are the one to help me with a solution!
Real qucik for clarification purposes, my weekly sales are currently summarized. Are you suggesting I create a new measure, something like Weekly Sales= (SumX('Weekly Sales' [Weekly Sales])
Then proceed and create a new formula for my Difference between Current Week and Prior Week something along the lines of Diff TW and LW = SumX('Weekly Sales',
IF(ISBLANK('Weekly Sales'[Weekly Netchange]) || ISBLANK('Weekly Sales'[Last Week Sales]),
BLANK(),'Weekly Sales'[Weekly Netchange] - 'Weekly Sales'[Last Week Sales]))?

Thank you for your clarification!