7) the answer is D. You need to know that oxygen gas is O₂ which means that A and B can't be right. Then you need to see which one is balanced and since D is and C is not, the correct answer has to be D.
8) You need to know that 1 mole of anything contains 6.02×10²³ particles. therefore you multiply 6.02×10²³ by 3 to get 1.806×10²⁴ atoms.
9) You need to multiply the number of moles of carbon dioxide by the molar mass of carbon dioxide (44g/mol). 2.1mol×44g/mol=92.4g. Therefore the answer is 92.4g of CO₂.
I hope this helps. Let me know if anything is unclear.