Trying to Calculate the difference between numbers in same column if 2 conditions are met

Hey, so I'm having a hard time with this column and hope I can get some help for it on here.

Ive got one column which is basically the net revenue, and another column which says if the row represents a control group or a test group, and lastly a column which is basically an index column which just matches control groups with test groups. To clarify every control is matched up with 1 or more test groups and they are represented by the same index number.

What I'm trying to do is create a column that returns the incremental revenue of the test compared to the control and returns a blank value if it is the control. And to be honest I'm stuck, any help would be largely appreciated.