Density is given by the following equation:
p = m / V = mass in kg / volume in m^3
Volume of the cube = 6 cm * 6 cm * 6 cm = 0.06 m * 0.06 m * 0.06 m = 0.000216 m^3
Mass = 270 g = 0.27 kg
Density = mass / volume = 0.27 kg / 0.000216 m^3 = 1250 kg/m^3