Hello Farhana. The block of wood is 3cm on each side so it is a cube.
The volume of a cube is given by s^3. So the volume of this block is 3cm x 3cm x3 cm = 27 cm^3.
density = mass/volume =27 g / 27 cm^3 = 1 g/cm^3.
the answer is 1 g/cm^3. I hope this helps!