My guess is that you need to work out the density of the coin- you know that it is 2.1cm3 and weighs 18.82g. Then you need to compare it to the density of gold, which should be quoted to you. A source that I found says pure gold has a density of 19.3gcm-3.
I found the density of the coin to be 8.962gcm-3 which is signifacantly below that of pure gold.
Funilly enough copper happens to have a density of 8.96gcm-3 so I would guess that was the main constituent of the coin (which would probably be gold coated, which is why the density is only slightly higher).