Simplify the following text for a Grade 6 native English-speaking person.
Original: The last independent Duke of Guelders was Adolf 's son Charles of Egmond ( 1492-1538 ) , who , backed by France , expanded his realm further north , to incorporate what is now the Province of Overijssel .
Simplified: The last independent Duke of Guelders was Adolf 's son Charles of Egmond ( 1492-1538 ) .