Help with a thermo derivation?

Hi all, first post so I hope this is ok to go here. It's not a homework question, just something in my book I don't get...

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 = -RT2/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
 
Nevermind, I went about it by dH/dP = d(U+PV)/dP and it worked out. If an admin can delete this, that would be great.
 

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