Hey there!
SiO₂ + HF → SiF₄ + H₂O
The problem here is that there is not an equal amount of O, H, or F atoms on both sides of the equation.
So let's balance!
Balance F.
1 on the left, 4 on the right. Add a coefficient of 4 in front of HF.
SiO₂ + 4HF → SiF₄ + H₂O
Balance H.
4 on the left, 2 on the right. Add a coefficient of 2 in front of H₂O.
SiO₂ + 4HF → SiF₄ + 2H₂O
Balance O.
2 on the left, 2 on the right. Already balanced.
Balance Si.
1 on the left, 1 on the right. Already balanced.
Our final balanced equation: SiO₂ + 4HF → SiF₄ + 2H₂O
Hope this helps!