One very common thing when using Excel formula is that you need to keep trying some formulas before you achieve the final results. This will need to enter a formula and you keep editing it till the ideal results:
Step 1: Double click the cell in the worksheet that contains a formula (e.g., B2);
Step 2: Change the formula (either from the cell or from the formula bar), for example, change formula "=A1+A2-A3" to formula "=A1+A2";
Step 3: Press "Enter" from the keyboard to finish.