Answer:
 2H₂ + O₂ → 2H₂O 
Step-by-step explanation:
The expression of the equation is given as: 
 _H₂ + 2O₂ → 2H₂O 
Now for expression above, 
 Reactants Products 
 
H 2 4 
 O 4 2
to balance the equation, we use 2 moles of hydrogen gas and 1 mole of oxygen gas; 
  2H₂ + O₂ → 2H₂O