Writing a Balanced Equation? Chemistry?

First, balance the atoms from the molecules with the largest number of atoms. In this case, that is C3H8. For the moment, assume there will only be 1 C3H8 molecule to keep things easy. You can always alter it later, if you need.

C3H8 + ?O2 → ?CO2 + ?H2O

There are 3 C atoms on the left. You need 3 on the right, so you need 3 CO2 molecules.

C3H8 + ?O2 → 3CO2 + ?H2O

Next, balance the H atoms. There are 8 on the left. You need 8 on the right, giving you 4 H2O molecules.

C3H8 + ?O2 → 3CO2 + 4H2O

Now count the number of oxygen atoms on the right. There are 3×2 + 4 = 10 O atoms on the right. This will yield 5 O2 molecules on the left.

C3H8 + 5O2 → 3CO2 + 4H2O

Counting the atoms again, there are equal amounts on both sides of the arrow. Therefore, this equation is now balanced.