CH4 + 2O2 ==> CO2 +2H2O + heat
This is a combustion reaction; also an oxidation-reduction equation.
For #4, add up the molar masses on each side and see if they match.
You have 16 for CH4 and 64 for 2O2 for a total of 80 grams.
What do you have on the right? Does that prove the law of conservation of mass?