Maxwell relation with 3 variables?

[anony]

Hello,
I have the thermodynamic potential dG = -SdT + VdP - MdB and I find that

(dG/dB)_(T,P) = -M and (dG/dT)_(P,B) = -S, where I have used _(letters) to denote constants and that these are partial differentials. I want to prove the Maxwell relation that

(dS/dB)_(T,P) = (dM/dT)_(B,P) *

so I do

(d/dB (dG/dT)_(P,B))_(T,P) = (d/dT (dG/dB)_(T,P))_(P,B)

But then bring these partials together, I no longer no what it supposed to be kept constant, or how I get to *

Sorry, about the messiness, and cheers for any help. I didn't put this in the homework section because its a pretty general question rather that an actual problem.

EDIT: Nevermind, I'm being an idiot. I got it now.

 

[JANm]

' Have found my thermodynamics book and see that I understand What S,T,V and P are.
What are M and B here?