Final answer:
The density of the given cube is 0.5 g/cm³.
Step-by-step explanation:
The density of an object is calculated by dividing the mass of the object by its volume. In this case, the mass of the cube is 21 grams and its volume is given as 2 cm x 7 cm x 3 cm = 42 cm³. So, the density of the cube can be calculated as:
Density = Mass / Volume = 21 g / 42 cm³ = 0.5 g/cm³