|For the graph of $f$: Find $y=8$ on the Y axis, then find the
point on the graph of $f$ directly right (left) of that point,
$(20,8)$, determine it's first coordinate, $20$, and that is the
desired value for $x$.
|For the mapping diagram of $f$: Find $y=8$ on the Y axis ,
then find the focus point of $f$, $F= [\frac 1 2, -2]$ on the
mapping diagram. Draw the line through F and the point $y=8$, to
find the point of intersection of this line with the X axis, $x=20$,
which is the desired value for $x$.