I got 11e-29 for the first answer which I believe is correct. I just don't understand how to do part b.
PROBLEM:
Cobalt has a density of 8.85 g/cm3, and the mass of a cobalt atom is 9.79 ✕ 10−26 kg.
(a) If the atoms are spherical and tightly packed, what is the volume of a cobalt atom?
(b) What is the distance between the centers of adjacent atoms?
If by "tightly packed" you mean in a rectangular lattice, then the distance between centers is just the diameter of the atom. You have the volume, which is
v = 1/6 π d^3