What is X is this reaction? NaOH + X - -> NaCH3COO + H20

Since both sides have to be equal to each other, subtracting the left side from the right side will leave you with the other reactant.

On the right side, you have 1 sodium, 2 carbon, 5 hydrogen, and 3 oxygen.

If you take away 1 sodium, 1 oxygen, and 1 hydrogen you are left with 2 carbon, 4 hydrogen, and 2 oxygen.

This is written as CH3COOH

AKA Acetic Acid