a. \(2x^2+6x^2=x^2+3x\)
\(\Leftrightarrow7x^2-3x=0\)
\(\Leftrightarrow x\left(7x-3\right)=0\Leftrightarrow\left[{}\begin{matrix}x=0\\7x-3=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=0\\x=\frac{3}{7}\end{matrix}\right.\)
Vậy...
b. \(x^4+x^3+x+1=0\)
\(\Leftrightarrow x^3\left(x+1\right)+\left(x+1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x+1=0\\x^3+1=0\end{matrix}\right.\) \(\Leftrightarrow x=-1\)
Vậy...