Click the function name in the tip to get a Help topic on how to enter an argument.
The comma you typed after the value for the rate argument causes the nper argument name to appear in heavy dark letters. If you recalled the meaning of "nper," you would enter 18 (there are 18 monthly payments).
If you didn't recall the meaning of "nper," however, you could learn about it by moving the pointer to PMT on the tip. The pointer would change to a hand, and PMT would become a hyperlink. You would click that link to display the PMT Help topic, where you could read that the nper argument is "the total number of payments for the loan."
Next you could minimize that Help topic, move the insertion point to the worksheet, and enter a value for the nper argument: 18.
Note You can also get help with function arguments (after entering the equal sign, a function name, and the opening parenthesis) by clicking the Insert Function button on the formula bar . This button opens the Function Arguments dialog box, which provides both information and boxes for entering the function arguments.