Hey there!
N₂H₄ + O₂ → NO₂ + H₂O
First, balance N.
Two on the left, one on the right. Add a coefficient of 2 in front of NO₂.
N₂H₄ + O₂ → 2NO₂ + H₂O
Next, balance H.
Four on the left, two on the right. Add a coefficient of 2 in front of H₂O.
N₂H₄ + O₂ → 2NO₂ + 2H₂O
Lastly, balance O.
Two on the left, six on the right. Add a coefficient of 3 in front of O₂.
N₂H₄ + 3O₂ → 2NO₂ + 2H₂O
This is our final balanced equation.
Hope this helps!
