First go to
https://www.searspartsdirect.com
Then enter the model number, in your case 917.280330.
Next, click on the orangish color button that says Shop Parts.
Click on the ground drive area to get the craftsman/Husqvarna ground drive belt (part number 56 in the diagram).
Now, scroll through the parts until you see the ground drive belt, and there it is, Part Number 532125907.
Now go back to the top of the web page and scroll over to the right until you see the Mower Deck. Now press on Mower Deck and you will see that you are looking for part number 27 in the diagram. Next, go through the parts list and you will see that the part number for the mower deck drive belt is Part Number 419271.
Just as an FYI, the 917 prefix in the model number indicates that your mower was manufactured by Husqvarna (also known as American Yard Products or AYP).
Now that you have the part numbers for the belts, you can go to
www.jackssmallengines.com,
www.rcpw.com, or any one of a large number of online OPE parts stores.
Note, most of these parts sights will give you the choice to order an OEM belt or an aftermarket belt. I've had good luck with aftermarket belts from Sterns, Rotary, and Oregon, and I saved some money along the way.