Answer
$\text{a) increasing: }
(-\infty,4]
\\\text{b) decreasing: }
[4,\infty)
\\\text{c) constant: }
\text{none}$
Work Step by Step
From the $x-$interval of $
( -\infty, 4 ]
$ the graph goes up.
From the $x-$interval of $
[4,\infty)
$, the graph goes down.
Hence, the graph has the following characteristics at these intervals:
\begin{array}{l}\require{cancel}
\text{a) increasing: }
(-\infty,4]
\\\text{b) decreasing: }
[4,\infty)
\\\text{c) constant: }
\text{none}
.\end{array}