$M=50gmol^{-1}$
$z=2$
$d=5.91gcm^{-3}$
$V=a^3$
$N_A=6.023\times 10^{23}$
$d=\large\frac{z\times M}{N_A\times a^3}$
$a^3=\large\frac{z\times M}{N_A\times d}$
$\;\;\;\;=\large\frac{2\times 50}{5.91\times 6.023\times 10^{23}}$
$\;\;\;\;=2.809\times 10^{-23}cm^3$