If you are using Excel for the web click Home Number Format Percentage. A bedrock of marketing is the calculation of percent change. Navigate to your worksheet.
Then divide the difference by last years number.
This time we fix the reference to cell B2. Next divide the difference by last years number. Select cell C3 click on the lower right corner of cell C3 and drag it down to cell C13. One common way to calculate percentage change with negative numbers it to make the denominator in the formula positive.