Question

Write the balanced chemical equation for the reaction in which methane, CH4, reacts with oxygen gas to produce water and carbon dioxide.

Answer 1

CH4 + O2 -> H2O + CO2

Then balance:

CH4 + 2 O2 -> 2 H2O + CO2

Answer 2

First your write the equation correctly.

CH4 +O2 ----- CO2 + H2O

The number are subscripts.
Now count the number of each kind of atom on the left and right side of the equation. Note, you have 4 H on the left and as written you have only 2 on the right. So add a coefficient-- a number in front of the formula.
CH4 +O2 ----- CO2 + 2 H2O
With the added coefficient in front of water, the number of hydrogens balance but the number of oxygens do not. There are only 2 oxygens on the left and a total of 4 oxygens on the right. A coefficient of 2 in front of oxygen on the left balances the equation.