\(\left|x+1\right|+\left|x+4\right|=3x\)
\(\left\{{}\begin{matrix}\left|x+1\right|\ge0\\ \left|x+4\right|\ge0\end{matrix}\right.\) \(\Rightarrow\left|x+1\right|+\left|x+4\right|\ge0\Rightarrow3x\ge0\)
\(\Rightarrow x+1+x+4=3x\)
\(\Rightarrow2x+5=3x\)
\(\Rightarrow x=5\)