ĐKXĐ: \(x\ne\left\{-3;-1\right\}\)
\(\Leftrightarrow\frac{-x^2-3}{\left(x+1\right)\left(x+3\right)}=\frac{6\left(x+1\right)-3\left(x+3\right)}{\left(x+1\right)\left(x+3\right)}\)
\(\Leftrightarrow-x^2-3=6x+6-3x-9\)
\(\Leftrightarrow x^2+3x=0\Rightarrow\left[{}\begin{matrix}x=0\\x=-3\left(l\right)\end{matrix}\right.\)