Density is mass per unit of volume, usually measured in g/cm³. So first determine the volume of the cylinder:
v = π r ² h = π ((39.0 mm) / 2)² (39.0 mm)
v = 59,319/4 π mm³ ≈ 46,589 mm³ ≈ 46.589 cm³
Then the density is
ρ = m / v
ρ = (1 kg) / v = (1000 g) / v
ρ ≈ 21.464 g/cm³
Or, if you want to preserve the given units, that's
ρ ≈ 0.00002146 kg/mm³
