Get some sheet metal and pop rivots, car body filler sand paper and paint.
Cut off all the rusted area. Clean / sand it well. Cut a piece of sheet metal to slightly bigger size than the hole. Pop rivot it onto the body of your car. Take some body filler and the hardener (small doses, it dries fast) mix it up on a piece of cardboard. Spread it onto the edges of the sheetmetal. Let dry. Sand it down until it’s smooth. paint.
to keep rust from forming there again, wash your wheel wells out. Aim the hose up and get all the dirt, salt ect out of there.