Electrons can gain the energy it needs by absorbing light. If the electron jumps from the second energy level down to the first energy level, it must give off some energy by emitting light. The atom absorbs or emits light in discrete packets called photons, and each photon has a definite energy.
Very simply, you get twice as much Hydrogen as Oxygen, because the chemical formula is H2O, meaning two hydrogen atoms bonded to one oxygen atom.
Answer: Rate law is, R= 6.02[A]¹[B]²
Explanation in attached image
Nitrogen=2, Hydrogen=8, Carbon=1, Oxygen=3
Hydrogen=4, Carbon=2, Oxygen=2
Iron=1, Nitrogen=2, Oxygen=6