Hi,

As a measure:

MyMeasure =
VAR ThisWeek =
    MIN( 'Table'[Week] )
VAR PreviousWeek =
    CALCULATE(
        MAX( 'Table'[Week] ),
        FILTER( ALLSELECTED( 'Table' ), 'Table'[Week] < ThisWeek )
    )
VAR ThisSales =
    MIN( 'Table'[Sales] )
VAR PreviousSales =
    CALCULATE(
        MIN( 'Table'[Sales] ),
        FILTER( ALLSELECTED( 'Table' ), 'Table'[Week] = PreviousWeek )
    )
RETURN
    ThisSales - PreviousSales

Regards