GIVEN :-

Lattice constant, a = 2.87 A⁰ = 2.87 x 10^-10 m.
Lattice system = BCC
Density = 7870 kg / m³
N = 6.02 x 10²⁶ atoms / k. mol.
For BCC, n = 2 atoms

Density = nM / N (a)³
So, atomic mass, M = (Density) N (a)³ / n
i.e. M = [(7870 kg/m³) (6.02 x 10²⁶ atoms / k. mol) (2.87 x 10^-10 m)³] / 2 atoms
i.e. M = 56 kg / k.mol