If it is the drive belt the only way to diagnose the cause is to remove the deck and then crawl under and look for bad tensioner arm, bad pulleys and worn guides. Deck will need to be pulled to replace the belt anyway. If you post the model and serial numbers, under the seat, we will be able to tell you the belt part number and maybe even a procedure for replacing the belt. While the deck is off it would not be a bad idea to do all the checks which mechanic mark listed.
https://www.amazon.com/Replacement-Craftsman-Poulan-140294-532140294/dp/B07GCM1CR8
remove mower deck, take pictures if you wish to facilitate reinstallation, check idler pulleys for bearing wear & some have grease fittings, take your time & clean, inspect, & grease deck spindles before re-installing, check blades after cleaning them to see if they need to be replaced or dressed up with a file & properly balanced before re-installing them.
https://www.google.com/search?q=pou...ome..69i57.33994j0j4&sourceid=chrome&ie=UTF-8
Thank you Rivets,Here is the owners manual which shows how to replace the drive belt.
https://www.manualslib.com/manual/132725/Poulan-Pro-161491.html?page=22#manual