Density is mass/volume. You want the density in gm /cm3.
One way to do this is to change the dimensions to cm by multiplying kg by 1000 and change meters to cm by multiplying by 100.
The formula for the volume of a sphere is 4/3 x pi x r3 = 4/3 x 3.14 x 903 = 3.05 x 106 cm3
Density = 900kg x 1000 g/kg / 3.05 x 106 cm3 = 0.3 g/cm3