What is the density of a block of wood measuring 9cmX2cmX6cm with a mass of 5.4g?

Density = mass/volume = 5.4g/(9*2*6) cm3 = 5.4/108 cm3 = 0.05 g/cm3.

Balsa, one of the least dense woods, has a density of 0.16 g/cm3 which is more than 3 times the "density" of your wood! So have you discovered a super-light wood species?