Calculate the mass of a 3.0 cm3 sample of iron ( whose density is equal to 7.87 g/cm3 )

We can use a simple equation to solve this problem. 
    d = m/v

Where d is the density, m is the mass and v is the volume.

d = 7.87 g/cm³
m = ?
v = 3.0 cm³

By applying the equation,
    7.87 g/cm³ = m /  3.0 cm³
                  m = 7.87 g/cm³ x 3.0 cm³ 
                  m = 23.61 g

Hence, the mass of 3.0 cm³ of iron is 23.61 g.