if column B includes the text "total" exactly, then the following
formula would work (copied to all rows):
=IF(B1="total",F1-E1,"")

however, if by "contains" you mean that the cell in B3 might have the
word "SubTotal" or "Sub Total" or even "Total widgets", then you will
need:
=IF(ISERROR(FIND("total",LOWER(B1))),"",F1-E1)