Hello @lusilva, I think you will want to use the Lag function in QuickSight. This will allow you to aggregate the average for the week previous and add it to the same row as the average for the current week. Then, you can subtract the 2 values once they exist on the same row.
I will mark this as a solution for now, but if you have any questions or if the function doesn’t work as expected please let me know!