Help with formatting rule expression

[MauriceWhelan]

Hi. I have a table Sales_Orders and a related table Sales_Orders_Line_Items. I am trying to add a conditional expression on a Format Rule that would highlight ‘Product Lines’ in a Sales_Orders_Line_Items table if the quantity has been changed from the original quantity entered. I know there is a [_THISROW_BEFORE] & [_THISROW_AFTER] function but that is just giving me a ‘Unable to find column’ error. Can this be done and if so could someone advise on the right conditional expression?

I have attached a screenshot of a detail view.

Thank you.