a. \(x\left(x+7\right)=0\Leftrightarrow\left[{}\begin{matrix}x=0\\x+7=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-7\end{matrix}\right.\)
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c. \(x^3-49x=0\Leftrightarrow x\left(x^2-49\right)=0\Leftrightarrow\left[{}\begin{matrix}x=0\\x+7=0\\x-7=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-7\\x=7\end{matrix}\right.\)
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d. \(x^2-81=0\Leftrightarrow\left[{}\begin{matrix}x+9=0\\x-9=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-9\\x=9\end{matrix}\right.\)
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