I am looking through my thermodynamics textbook (Smith,Van Ness, and Abbot, Intro to Chemical Eng thermodynamics) and they give the eq (6.19):

([tex]\partial[/tex]H/[tex]\partial[/tex]P)

_{T}= V - T([tex]\partial[/tex]V/[tex]\partial[/tex]T)

_{P}

then, "because V = ZRT/P we can write more concisely:

([tex]\partial[/tex]H/[tex]\partial[/tex]P)

_{T}= -RT

^{2}/P (([tex]\partial[/tex]Z/[tex]\partial[/tex]T)

_{P})

Can anyone explain to me how they did that? I'm sure there is just some simple substitution I am missing