It's a 2 WD...take the wheel off and then, so you can easily get to the backside, take the hub off if it's shrouded while on the tractor. It's likely to have a bit of the bolt sticking through on the backside. You can drill it out from the backside with a regular right hand twist bit and hope it catches and screws it out or you could cut a slot in the end of the bolt if it sticks out a ways from the hub and try a big screw driver that has a hex on the shank that you can put a wrench on for leverage. Vise Grips might also work but will chew up the shank. Wire wheel the threads first, lube them and then try to turn it so it backs out the way it was threaded in if it has a buggered up spot where it broke or if it's ok try to screw it out the back side of the hub.