In another cell:
1. type an equals sign (=)
2. Click on the first cell
3. Type a hyphen, or minus sign (-)
4. Click on the second cell
5. Hit enter.
So if your two cells were B2 and B3, you'd get
=B2-B3
If after hitting ENTER, you see the formula, not the
answer, then change the formatting of the cell to general,
and try again. (Format/Cells/General)
Bill