For the graph of $f$: Find
$y=20$ on the Y axis , then find the point on the graph of
$f$ with second coordinate $20$, determine it's first
coordinate, $6$, and that is the desired value for $x$. 
For the mapping diagram of $f$:
Find $y=20$ on the target axis , then find $x$ on the source
axis with the function arrow pointing to $20$. 
To do this, look for the point where the line
$y=20$ intersects the line that is the graph of $f$ 
To do this, find the focus point of
$f$, $F= [5, 10]$ on the mapping diagram. [Use Geogebra?]
Draw the line through F and the point $y=20$, to find the
point of intersection of this line with the X axis, $x=6$,
which is the desired value for $x$.
