What number should be substituted for the question mark to balance the equation and why is that the correct number?

2c2h6 +? o2 → 4co2 + 6h2o + heat

Which of the following best describes internal energy? a. the difference between the kinetic and potential energies of the particles in a system b. the sum of the kinetic and potential energies of the particles in a system c. the sum of the kinetic and thermal energies of the particles in a system d. the difference between the kinetic and thermal energies of the particles in a system