Let $f: \mathbf{R} \rightarrow \mathbf{R}$ be defined as,
$$
f(x)= \begin{cases}-55 x, & \text { if } x<-5 \\ 2 x^3-3 x^2-120 x, & \text { if }-5 \leq x \leq 4 \\ 2 x^3-3 x^2-36 x-336, & \text { if } x>4\end{cases}
$$
Let $A=\{x \in R: f$ is increasing $\}$. Then $A$ is equal to:
