There is no actual function for subtraction like the SUM() function is for
addition.
Simply use a minus sign [ - ].
Subtract Column A from Column B by entering this formula in Column C:
=B1-A1
You can then click in C1 again, and you'll see a small square in the lower
right corner of the cell.
This is called the "fill handle".
Hover the cursor over this square until the cursor changes from a fat white
cross to a skinny black cross.
Then click, and drag this down the column as far as needed to copy the
formula to every cell down Column C.
If you're going to add and subtract in the same formula:
=B1-(A1+A2+A3+A4)
OR
=B1-SUM(A1:A4)
In XL, there is almost always more then one way to perform calculation.