Material science: Imperfections in solids

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To determine the number of vacancies per cm³ needed for a BCC iron crystal to achieve a density of 7.87 g/cm³, first calculate the number of atoms in a unit cell, which for BCC is 2. The volume of a unit cell can be found using the lattice parameter (2.866x10^-8 cm), resulting in approximately 6.5x10^-23 cm³ per unit cell. Using Avogadro's number, there are about 2.3x10²² unit cells in 1 cm³. The average mass of an iron atom is 9.27x10^-23 g, leading to a theoretical mass of 21.5 g/cm³ for pure iron, indicating that vacancies are necessary to reach the target density of 7.87 g/cm³.
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Determine the number of vacancies per cm^3 needed for a BCC iron crystal to have a density of 7.87 g/cm^3. The lattice parameter of the iron is 2.866x10^-8 cm and the atomic weight of the iron is 55.847 g/mol. Given: Avogradro's number, N=6.022x10^23 atoms/mol

Help please? i have no idea how to solve this question :( I am gona have my first test for material science on wednesday!
 
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How many atoms in a unit cell? How many unit cells in 1 cm3? What is the average mass of an iron atom?

What is the theoretical mass of a cm3 of Fe?
 
theyre not given:( that is why I am having a hard time solving the question now sir
 

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