C6H12O6 is glucose, a sugar. If it is completely combusted, the only two products will be carbon dioxide and water. Oxygen will also be present from the atmosphere. The unbalanced equation is shown below.
C6H12O6 + O2 --> CO2 + H2O
The six carbons from the glucose will all go to CO2, so we need to add a six there.
C6H12O6 + O2 --> 6CO2 + H2O
The twelve hydrogens from the glucose will all go to H2O, so we need to add a six there.
C6H12O6 + O2 --> 6CO2 + 6H2O
Now we need to balance out the oxygens. There 6 from the glucose on the left and a total of 18 on the right. So we need twelve more on the left, which means we need a six in front of the O2.
C6H12O6 + 6O2 --> 6CO2 + 6H2O
That is the balanced equation.