f(x)={\begin{matrix}-55x, & \mathrm{if}x<-5 \\ 2{x}^{3}-3{x}^{2}-120x, & \mathrm{if}-5\leq x\leq 4 \\ 2{x}^{3}-3{x}^{2}-36x-336, & \mathrm{if}x>4\end{matrix}
f'(x)={\begin{matrix}-55, & \mathrm{if}x<-5 \\ 6{x}^{2}-6x-120, & \mathrm{if}-5\leq x\leq 4 \\ 6{x}^{2}-6x-36, & \mathrm{if}x>4\end{matrix}
{f}^{'}(x)={\begin{matrix}-55; & x<-5 \\ 6(x-5)(x+4); & -5<x<4 \\ 6(x-3)(x+2); & x>4\end{matrix}
f(x) is increasing
⇒f′(x)>0
So, x∈(−5,−4)∪(4,∞)