$6-4$ Apply the Gibbs$/$Duhem equation ${\displaystyle \underset{?}{\overset{?}{}}}{x}_i\frac{d}{b}ar(M)x_i=(\frac{\text{d}\text{e}\text{l}\text{M}}{dT}{)}_{P,x}dT+(\frac{\text{d}\text{e}\text{l}\text{M}}{dP}{)}_{T,x}dP$ to a binary solution at constant $T$ and $P,$ and give graphical interpretations of the results.