Answer
$a.\quad 4$
$b.\quad-1$ and $3$
Work Step by Step
The graph of a function is the set containing points $(x,f(x))$,
where x is an element of the domain, and f(x) is the corresponding element of the range,
meaning that we use the horizontal axis for $x$, the domain,
and the vertical axis for $y=f(x)$, the range.
$ a.\quad$
$(1,4)$ is a point on the graph, so $f( 1)=4$
$ b.\quad$
$(-1,2)$ is a point on the graph with second coordinate $2.$
$(3,2)$ is a point on the graph with second coordinate $2.$
So, $x=-1$ and $x=3$ are values for which $f(x)=2.$