Hi 

This is Kelda Park writing. :) 

You are asking that the part called the hydrolysis of salt, right? 

Do you understand the concept of the hydrolysis of salt? 

That's quite hard compared to the level of other SAT chem. topic, anyways, salts (neither acid nor base we learn) have also some acidic or basic properties. 

That's why their pH values are not all 7 at the equivalence point. 

So you will learn this topic in depth in AP Chemistry. 

However, if you understand the salt Na3PO4 is basic, that is simply because of that salt being resulted from NaOH(strong base) and H3PO4(weak acid), right? 

That's not a proper or thorough reasaon, but anyways, strong base and weak acid produces the basic salt like Na3PO4. 

More fundamental reason is this, PO43- + H2O --> HPO42- + OH-

which means, PO43- can work as a base in water solution. 

When Na3PO4 dissolves in water, it will dissociate fully (strong electrolyte and very soluble), making PO43- ions as well. 

Therefore, Na3PO4 is basic due to the PO43- ions in water, just like the equation above about what PO43- behaves in water. 

Do you get it? 

It IS a hard question, so you dont have to be worried about, it will always comes out with some kind of logical connection like this question. 

The latter statement will be connected anyhow to the previous one, so that you can make a logical guess. 

I hope it helps. 

Best, 

Kelda Park