# How many atoms of oxygen are in one formula unit of Ca(NO_3)_2?

$N {O}_{3}$ has 3 atoms of oxygen in it per molecule (indicated by the subscript of 3) and ${\left(N {O}_{3}\right)}_{2}$ means you have 2 $N {O}_{3}$ molecules, meaning you double the subscript to get 6 oxygen atoms.
Following the same rules, there is 1 atom of nitrogen in 1 $N {O}_{3}$ molecule (subscripts of 1 are not given) and 2 atoms of nitrogen in 2 $N {O}_{3}$ molecules.