Winger Lloyd Chalk and fly half Gareth Thompson also touched down for the home side in the first half with Thompson rattling up 13 points before making way for Withers.
In response flanker Daniel Thomas (twice) and centre Dylan Morgans crossed for the Scarlets with Jack Maynard getting the rest with his boot.
Earlier, Dragons Under 18 made the most of a home advantage at Newbridge by scoring 20 unanswered points against RGC 1404.
Dan Goode was first across the try line in the third minute which Niall Martin successfully converted. The Dragons kept momentum going throughout the first half with prop Elliot Dee scoring another try.
Martin ensured there wasn't a hint of a north Wales comeback by claiming a second half penalty and a try.