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