Metal M crystallizes into a FCC lattice with the edge length of 4 ....

Given ,$a = 4 \times {10}^{- 8}cm$

$d = 9 . 03g / ml$

$d = \dfrac{ZM}{{N}_{A}{a}^{3}}$

Volume of a unit cell = a³, where a is the edge length

d is the density of the solid substance and M is the molar mass.

Mass of the unit cell = number of atoms in unit cell × mass of each atom =$Z \times m$

(Here z is the number of atoms present in one unit cell and m is the mass of a single atom)$M = \dfrac{9 . 03 \times 6 . 02 \times {10}^{23} \times 64 \times {10}^{- 24}}{4} = 86 . 97g / mol$