On the right side of the equation I have 3 calciums,

I have 2 phosphorus, because I have 2 times 1 for 2.

And for oxygen I have 8 because I have 2 phosphates, and

each phosphate has 4 oxygens for a total of 8.

Now, in order to balance this, I have to make sure that

everything has the same number of atoms on the, both sides of the equation.

One thing I notice is that oxygen is present in two of my reactants.

And so I'm actually going to save that one for last in hopes that

balancing the calcium and the phosphorus will actually balance out the oxygen.

So the first thing I'm going to do is add in my coefficient for

calcium because I want to balance that.

So I need a three in front of the calcium oxide,

when I do that changes the calcium from one on the left to three on the left.

It also changes my oxygens.

Now I have 3 oxygens and 10 oxygens, for a total of 13 oxygens.