How do you balance #C_7H_14 + O_2 -> CO_2 + H_2O#?
The products of complete combustion of ANY hydrocarbon are carbon dioxide and water. Above I balanced the carbons, and then the hydrogens, and then the oxygens. Is it balanced?
Note that you use this same principle of balancing, of stroichiometry, whenever you make a cash or electronic transaction. Garbage in must equal garbage out.