$$\left. \begin{array} { l } { ( x ) = 6 x ^ { 3 } - 50 x ^ { 3 } - 120 } \\ { \text { a . R e n t t h e d e r c t i a t i o f e n t h e f u n c t i o \right.$$
$x=\frac{1089^{\frac{2}{3}}\left(\sqrt[3]{\sqrt{141134433}-11880}-\sqrt[3]{\sqrt{141134433}+11880}\right)}{2178}\approx -1.391727157$
