Can someone please explain and show me how to di this question please....

Jacy bought a gold necklace weighing 21.3 grams from the flea market. She filled a graduated cylinder with water to the 5.0mL mark and dropped her necklace in. The level in the  graduated cylinder rose to 704mL. is her necklace real or fake? If not, identify the possible composotion of her necklace.

Gold: 19.3 g/cm^3

coppert: 8086 g/cm^3

bronze: 9.87 g/cm^3

Any help would really be appreciated. Thank you