x1=1
x2=\(\dfrac{-2}{7}\)
\(21x^3-15x^2-6x=0\\ \Leftrightarrow x\left(21x^2-15x-6\right)=0\\ \Leftrightarrow x\left[\left(21x^2-21x\right)+\left(6x-6\right)\right]=0\\ \Leftrightarrow x\left[21x\left(x-1\right)+6\left(x-1\right)\right]=0\\ \Leftrightarrow x\left(x-1\right)\left(21x+6\right)=0\\ \Leftrightarrow\left[{}\begin{matrix}x=0\\x=1\\x=\dfrac{-2}{7}\end{matrix}\right.\)
\(21x^3-15x^2-6x=0\)
\(\Leftrightarrow21x^3-21x^2+6x^2-6x=0\)
\(\Leftrightarrow21x^2\left(x-1\right)+6x\left(x-1\right)=0\)
\(\Leftrightarrow\left(21x^2+6x\right)\left(x-1\right)=0\)
\(\Leftrightarrow3x\left(7x+2\right)\left(x-1\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}3x=0\\7x+2=0\\x-1=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=0\\x=-\dfrac{2}{7}\\x=1\end{matrix}\right.\)
