movement of last two entries

[drawing.blanks]

please see the attachment sheet. Wonder how I calculate the movement of the last two items.

For example, for week 6 report the result in AA is week 6 value - week 5 value ie 26-24 for Dept A; 15-22 for Dept B etc
For example, for week 7 report the result in aa is week 7 - week 6
For example, for week 8 report the result in aa is week 8 - week 7

any help would be much appreciated.

drawing blanks