What is the mass, in kilograms, of 14.0 l of gasoline? ( assume that the density of gasoline is 0.680 g/cm^3)

The answer is 9.52 kg.

The density (D) of is quotient of mass (m) and volume (V):
D = m/V

It is given:
m = ?
D = 0.680 g/cm³
V = 14 L = 14 dm³ (1 L = 1 dm³) = 14000 cm³
 
Since D = m/V, then m = D*V
m = 0.680 * 14000
    = 9520 g
    = 9.52 kg