Answer:
One CO2 (carbon dioxide) molecule should be added to the products side.
Step-by-step explanation:
The balanced equation is CH4 + 2O2 --> CO2 + 2H2O.
You have to make sure that there are the same number of carbon, hydrogen, and oxygen atoms on both sides of the balanced equation to make sure it obeys the conservation of matter.
The balanced equation I provided for you shows 1 atom of carbon on both sides, 4 atoms of oxygen on both sides, and 4 atoms of hydrogen on both sides.