The only thing that changes is pressure and volume. And so suppose we have P ideal and V ideal. Now, from this information we know that our P real is smaller than our P ideal. And that means we have to add some term to our P real to equate that to our P. Likewise, our Vreal is larger than the ideal and that's why we have to subtract some term from it, some term Y to get the ideal. And so, from experiments, scientists found out what this X and what this Y was. This X is N squared times A over V. Two this Y is N times B.