First find the volume of the cubic lattice given edge length of 646 pm or 6.46x10^-8 cm.
volume = (6.46x10^-8 cm)^3
volume = 2.7x10^-22 cm^3
The total mass of the lattice is:
mass = (5.850 g/cm^3) * 2.7x10^-22 cm^3
mass = 1.577x10^-21 grams
The molar mass of tin is 118.71 g/mol and the Avogadros number is 6.022 x 10^23 atoms/mol, hence:
Sn atoms = [1.577x10^-21 g / (118.71 g/mol)] * 6.022 x 10^23 atoms/mol
Sn atoms = 8 atoms
