The given compound is Calcium Nitrate, which is formed by combing Calcium, Oxygen and Nitrogen atoms.
In the compound [tex]Ca(NO_3 )_2[/tex],the subscript, 3 implies that, there are three atoms of oxygen in the parenthesis.
The 2 outside the parenthesis tells us the nitrate[tex](NO_3)[/tex] is doubled and that each atom is also doubled.
In order to obtain the number of oxygen atoms in the compound, we have to multiply the subscript, [tex]3[/tex] of oxygen, by the subscript [tex]2[/tex] which is outside the parenthesis.
This implies that,
The number of oxygen atoms [tex]=3\times2=6[/tex]
Therefore there are [tex]6[/tex] oxygen atoms in the given compound.
