The mathematical definition of density is the mass divided by the volume. In this case the volume of the cube is 6 x 6 x 6 = 216 cubic centimetres. Dividing a mass of 137.8g by a volume of 216 cubic cm yields a density of 0.6379 grams per cubic cm. To the nearest thousandth we need to round to 4 decimal places and so have a final answer of 0.638 grams per cubic cm.