C1 Đặt $\sqrt{6-x}=a;\sqrt{7+x}=b$
$\Rightarrow \begin{cases}a^2+b^2=13 \\ a+b+ab=11 \end{cases}\Rightarrow a^2+b^2+2ab+2a+2b=35$
$\Leftrightarrow (a+b)^2+2(a+b)=35$
$\Leftrightarrow \left[ {\begin{matrix} a+b=5\Rightarrow \begin{cases}a+b=5 \\ ab=6 \end{cases}\\ a+b=-7\Rightarrow \left\{ \begin{array}{l} a+b=-7\\ ab=18 \end{array} \right.(vô nghiệm) \end{matrix}} \right.$
$\Leftrightarrow \left[ {\begin{matrix} a=3\\ a=2 \end{matrix}} \right.$
$\Leftrightarrow \left[ {\begin{matrix} x=2\\ x=-3\end{matrix}} \right.$