Subtraction (-)

1. Click on the cell where you want the difference to be shown with the mouse pointer to make it an active cell.

2. Type the equal sign ( = ) in the cell to begin the formula
    * Formulas in Excel always begin with the equal sign.

3. Click on the cell of the minuend with the mouse pointer to add that cell reference to the formula or just type the name of the cell after the equal sign.

4. Type a minus sign ( - ) in the cell after the cell reference.

5. Click the cell of he subtrahend with the mouse pointer to add that cell reference to the formula or just type the name of the cell after the minus sign.

6. Press the ENTER key on the keyboard to show the answer.




source:
spreadsheets.about.com