The density of silver is 10.5 g/cm3. what is its density in ng / (mm) 3

First we convert each of the units separately (easier in calculation):

1 gm = 1000000000 n gm

1 cm^3 = 1000 mm^3

Then we get the requirement:

10.5 g/cm3 x (1000000000 / 1000) = 10500000 ngm/mm3