If $\displaystyle -2 <= x <= 1$, use the triangle inequality to show that $\displaystyle |2x^3 - 4x^2 +3x - 1| <=39$
2. $\displaystyle \begin{gathered} - 2 \leqslant x \leqslant 1\, \Rightarrow \,\left| x \right| \leqslant 2 \hfill \\ \left| {2x^3 - 4x^2 + 3x - 1} \right| \leqslant 2\left| x \right|^3 + 4\left| x \right|^2 + 3\left| x \right| + 1 \hfill \\ \end{gathered}$