\[\begin{array}{l}
f\left( x \right) = 0 \Leftrightarrow \left[ \begin{array}{l}
x = - 4\\
x = - 1\\
x = 2
\end{array} \right.\\
De\,thay\,cac\,nghiem\, - 1, - 2, - 4\,khong\,la\,nghiem\,cua\,tu\,thuc.\\
Vay\,dths\,co\,tat\,ca\,3\,duong\,tiem\,can:x = - 1;x = - 2;x = - 4
\end{array}\]