Answer is: the density of the material is 0.8 g/cm³.
m(material) = 6.75 g; mass of unknown material.
V(material) = 9 cm³; volume of unknown material.
d(material) = m(material) ÷ V(material).
d(material) = 6.75 g ÷ 9 cm³.
d(material) = 0.75 g/cm³; density of unknown material.