show how CH4 +O2=CO2+H2O obeys the law of conservation of mass

Answer 1

You don't have any numbers; however, we can substitute grams from moles.
CH4 + 2O2 ==> CO2 + 2H2O
On the left CH4 = 16 and 2O2 = 64 for a total of 80g.
On the right we have CO2 = 44 and 2H2O = 36. 44 + 36 = 80.