Welcome to the forum
You can use a simple subtraction formula for that, and just copy it down the column.
If the issue is that you don't want a column of zeros when there aren't figures in columns A & B, then you could use an IF function like this:
=IF(OR(A1<>"",B1<>""),A1-B1,"")
Just copy it down the entire column and it will only perform the subtraction if there is a number in either one or both of the columns.
Hope this helps!