2012-II-05

Ans: B
$\left\{\begin{array}{ll}
m+2n+6=7 & \ldots \unicode{x2460} \\
2m-n=7 & \ldots \unicode{x2461}
\end{array}\right.$

From $\unicode{x2460}$, we have

$\begin{array}{rcl}
m + 2n + 6 & = & 7 \\
m & = & 1-2n~\ldots \unicode{x2462}
\end{array}$

Sub. $\unicode{x2462}$ into $\unicode{x2461}$, we have

$\begin{array}{rcl}
2(1-2n)-n & = & 7 \\
2- 5n & = & 7 \\
n & = & -1
\end{array}$