The density of the lead is 11.3 g/cm³.
V = lwh = 30 cm × 20 cm × 1 cm = 600 cm³
Density = 6800 g/600 cm³ = 11.3 g/cm³
Note: The answer can have only one significant figure because that is all you gave for the height of the block. However, I calculated the density to three significant figures.