Solution:
\begin{parts}
\part [6] Please use mainly two particular equations (please write down the names of these two equations, with specified conditions)
\part [8] to derive and solve for $\displaystyle \overline{M}_1 = M + x_2 \frac{d M}{d x_1}$ and $\displaystyle \overline{M}_2 = M + x_1 \frac{d M}{d x_1}$
\end{parts}
