If you use Office Assistant, the answer to your query can be found by typing 'solver' and clicking Search. From the resulting options, select 'Define and solve a problem using solver'. Follow the instructions there.
Here's a summary of the instructions -
1. Select 'Solver' from the Tools' menu.
2. Make sure your target cells contain a formula.
3. Enter a name or cell reference for the target cell in the 'Set Target Cell' box.
4. If you want the largest possible target cell click Max, and click Min for the smallest.
5. Enter the reference or name for your adjustable cells in the 'By Changing Cells' box.
6. If you want Solver to propose the adjustable cells click 'Guess'.
7. Click Solve.
8. In the Solver Results box, click Keep Solver Solution. This will keep the solution values on the worksheet.
If you can follow this, you're far more adept with spreadsheets than I am. Sorry if this doesn't make much sense, but it's way beyond my skills with Excel.