Since you know PV= RT where R is the gas constant, with P, V and T variables, you know that
[tex]\frac{{PV}}{T} = R = {\rm{a}}\;{\rm{constant}}[/tex]
We can rearrange this
[tex]\left( {\frac{{PV}}{T}  R} \right) = {\rm{0}}[/tex]
Isn't this now in the format you seek?
However noting your other threads about Van der Waal's equation I wonder if your book was leading up to some more complicated function of P, V and T such as VDW.
Incidentally the answer to your question about P, is that P is the real pressure exerted by the gas, not some equivalent pressure of an ideal or other gas.
