Weight = Volume * Density
Weight = length*breadth*thickness* Density
length =12000 mm = 12 m
breadth =4390 mm = 4.39 m
thickness = 20 mm = 0.02 m
Density of steel = 7850 kg/m^3
[ Different steels have different Densities ]
Weight = 12 * 4.39 * 0.02 *7850 kg
[m^3 * kg/m^3 =kg ]
Weight = 8270.76 kg. is the weight of the steel sheet