\(x\left(x-1\right)=-x\left(x+3\right)\)
\(\Leftrightarrow x\left(x-1\right)+x\left(x+3\right)=0\)
\(\Leftrightarrow x\left(x-1+x+3\right)=0\)
\(\Leftrightarrow x\left(2x+2\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\2x+2=0\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-1\end{matrix}\right.\)
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