Hey there!
Si₂H₃ + O₂ → SiO₂ + H₂O
Balance H.
3 on the left, 2 on the right. Add a coefficient of 2 in front of Si₂H₃ and a coefficient of 3 in front of H₂O.
2Si₂H₃ + O₂ → SiO₂ + 3H₂O
Balance Si.
4 on the left, 1 on the right. Add a coefficient of 4 in front of SiO₂.
2Si₂H₃ + O₂ → 4SiO₂ + 3H₂O
Balance O.
2 on the left, 11 on the right. We can't do this by simply adding coefficients to O₂ since 11 isn't divisible by 2. So we need to multiply the coefficients in front of everything else by 2 first.
4Si₂H₃ + O₂ → 8SiO₂ + 6H₂O
Now we can balance O properly.
2 on the left, 22 on the right. Add a coefficient of 11 in front of O₂.
4Si₂H₃ + 11O₂ → 8SiO₂ + 6H₂O
This is our final balanced equation:
4Si₂H₃ + 11O₂ → 8SiO₂ + 6H₂O
This means the coefficient for O₂ when the chemical equation is balanced is 11.
Hope this helps!