Answer
As shown on the graph:
(a) The function is increasing on $( -\infty ,4)$,
(b) The function is decreasing on $( 4, \infty)$.
Work Step by Step
Moving from left to right, the function $f$ is increasing for x-values up to $x=4$
and decreasing for all x-values larger than 4
So,
(a) The function is increasing on $( -\infty ,4)$,
(b) The function is decreasing on $( 4, \infty) $.
