Ta có: \(\left\{{}\begin{matrix}\left|x\right|\ge0\\\left|3x\right|\ge0\end{matrix}\right.\Rightarrow\left|x\right|+\left|3x\right|\ge0\)
Mà \(\left|x\right|+\left|3x\right|=0\)
\(\Rightarrow\left\{{}\begin{matrix}\left|x\right|=0\\\left|3x\right|=0\end{matrix}\right.\Rightarrow x=0\)
Vậy x = 0