The answer is 18.
This is obtained by multiplying the three oxygen atoms in NO₃, that are inside the parenthesis by the number two outside the parenthesis. The number 2 is there because it comes from associating with Fe ²⁺ .
3×2=6 oxygen atoms present in the formula.
Then multiplied by the 3 that's in front of the formula (that indicates how many molecules).
6 x 3= 18