This is just a simple stoichiometry problem. Remember, we always start with what we know, and here we know we have 3.45 moles of CO2. So when writing the equation start with that. Next we need to do a mole to mole comparison and here since they already gave us CO2 in moles we did’t need to do an extra conversion. You are going to multiply the moles of CO2 by a fraction, where on the bottom you put mol of CO2 and on top mol of O. If we are just talking about one molecule of CO2, then we say there is only 1 mole of CO2, but how many moles of Oxygen? 2 right? By doing this, you have canceled out moles of CO2 and now you have moles of O. But we want mass of O, and Im assuming they want it in grams. So take those moles of O and multiply by molecular weight of O. Make sure to put 1 mole of O on bottom, so moles will cancel out, and on top you put the weight of O. You should know that 1 mole of O weights 16 grams. When you do all the math, you should get 110.4 grams of O.