how many moles of oxygen are in one mole of aluminum carbonate?
Finally, to convert this to atoms of oxygen, use Avogadro's constant, which tells you that in order to have 1 mole of atoms of oxygen, you need to have 6.022⋅1023 atoms of oxygen. The answer is rounded to three sig figs, the number of sig figs you have for the mass of aluminium carbonate.