\begin{align*}
p &=  \frac{  \partial G}{\partial x}+ \frac{  \partial G}{\partial p} \frac{  \mathop{\mathrm{d}p}}{\mathop{\mathrm{d}x}}\\ 
p -  \frac{  \partial G}{\partial x}&= \frac{  \partial G}{\partial p} \frac{  \mathop{\mathrm{d}p}}{\mathop{\mathrm{d}x}}\tag{3A}
\end{align*}