Ferric oxide crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by ferric ions

Given, 

Ferric oxide crystalizes in a hexagonal close packed array of oxide ions.

Two out of every three octahedral holes are occupied by ferric ions.

Let the number of oxide ions = x

Since, two out of every three octahedral holes are fitted by ferric ions.

Thus, voids filled by ferric ions

(Fe3+) =2/3x

Therefore, number of ferric ions

(Fe3+) =2/3x

Now, the ratio of ferric ions to the oxide ions 

i.e. Fe3+ : O2- = 2/3x:3

= Fe3+ : O2- = 2/3:1= 2 : 3

Thus, formula of ferric oxide is Fe2O3 .