\(\left[x+8\right]=-3x\)
\(\Leftrightarrow\left[x\right]+8=-3x\)
Đặt \(-3x=y\in Z\Rightarrow x=-\dfrac{y}{3}\)
\(\Rightarrow\left[-\dfrac{y}{3}\right]+8=y\)
\(\Leftrightarrow\left[-\dfrac{y}{3}\right]=y-8\)
\(\Rightarrow y-8\le\dfrac{-y}{3}\le y-7\)
\(\Rightarrow\left\{{}\begin{matrix}4y\le24\\4y\ge21\end{matrix}\right.\) \(\Rightarrow21\le4y\le24\)
\(\Rightarrow y=6\)
\(\Rightarrow x=-2\)