To get a program such as Excel to subtract numbers or to carry out any other mathematical operation, you have to tell it what you want it to do using the correct instructions in the correct order.
These instructions are contained in a formula. The formula tells Excel, what data to use, the mathematical operation to carry out, such as subtraction, and where to place the answer in your spreadsheet.
This may sound rather complicated but it isn't really. To find out how easy it is, read the short tutorial on How to Subtract in Excel. This tutorial has links to other basic operations, such as addition, division, and multiplication.