At room temperature the electrical conductivity and the electron mobility for aluminum are 3.8 107 (.m)-1

Question:

At room temperature the electrical conductivity and the electron mobility for aluminum are 3.8 × 107 (Ω.m)-1 and 0.0012 m2/V.s, respectively.
(a) Compute the number of free electrons per cubic meter for aluminum at room temperature.
(b) What is the number of free electrons per aluminum atom? Assume a density of 2.7 g/cm3.