Ta có: 3x(x – 1) + x – 1 = 0

\( \Leftrightarrow \)3x(x – 1) + (x – 1) = 0

\( \Leftrightarrow \)(x – 1)(3x + 1) = 0

\( \Leftrightarrow \left[ \begin{array}{l}x - 1 = 0\\3x + 1 = 0\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}x = 1\\x = \frac{{ - 1}}{3}\end{array} \right.\).