Ta có:7x(3x-1)+21(3x-1)=0
=>(7x+21)(3x-1)=0
=>7(x+3)(3x-1)=0
\(\Rightarrow\left[{}\begin{matrix}7=0\left(loại\right)\\x+3=0\\3x-1=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-3\\3x=1\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x=-3\\x=\dfrac{1}{3}\end{matrix}\right.\)
Vậy \(x\in\left\{-3;\dfrac{1}{3}\right\}\)