18800 kg/m^3
This is a simple matter of multiplying by the appropriate conversion factors. So let's see what they are:
We have grams and want kilograms. That will be a simple matter of dividing by 1000.
We have cm^2 and want m^3. This is slightly more difficult. There's 100 cm per meter. And since we need to cube it, that will be 100^3 = 1000000.
So the result will be
18.8 g/cm^3 * 1000000 cm^3/m^3 / 1000 g/kg = 18800 kg/m^3
