For the graph of $f$: Find
$y=0$ on the Y axis , then find the point(s) on the graph of
$f$ with second coordinate $0$, determine it's first
coordinates, $2$ and $2$, and those are the desired values
for $x$. 
For the mapping diagram of $f$:
Find $y=0$ on the target axis , then find point(s) $x$ on
the source axis with the function arrow pointing to $0$. 
To do this, look for the point(s) on the $X$
axis, the line $y=0$, where the axis intersects the graph of
$f$ 
To do this, find the local extreme point of
the quadratic numerator of $f$ on the mapping diagram, $x =
0$. Move symmetrically above and below that value by $\pm
2$, which are the desired values for $x$.
