The volume of water displaced by an object is equal to its immersed volume.
Length=3m, breadth =2m , height(sinked into water)=1.5cm=0.015m
volume of water displaced= 3 * 2 * 0.015 = 0.090 m^3
mass= volume* density
density of water =1000 kg/m^3
mass= 0.090 * 1000 = 90 kg
which is the required answer.