You're given the istructions on the left of your screen: if you know the annual salary and you want the annual salary, you'll have to divide the annual salary by 12, since there are 12 months in a year.
If this is an excel worksheet, you'll have to fill your cell with a formula. Click on your formula, and write an equal sign "=". This will tell excel that you're about to fill this cell with a calculation.
Now click on the annual salary cell, and then type "/12". Press enter. Your cell should now contain the annual salary value divided by 12, and should be dynamically linked to that value. By this I mean that if you change the annual salary, the monthly salary will change as well.
For example, if the annual salary is the cell B3, your cell should contain "=B3/12"
