The problem can be restated now: To find a value for $x$ where $f(x) = g(x)$.

This problem and its solution can be visualized both on the graphs and the mapping diagrams with focus points for the linear functions $f$ and $g$.

For the graphs of $f$ and $g$: Find the first coordinate for the
point on the graph of $f$ which is also on the graph of $g$. |
For
the mapping diagrams of $f$ and $g$:Find $x$ on the source axis with the function arrow for $f$ coinciding with the function arow for $g$. |

To do this, look for the point where the lines that graph $g$ and $f$ intersect. The first coordinate of that point, $15$, is the desired value of $x$. | To do this, find the focus point of
$f$, $F= [5, -10]$ on the mapping diagram. and the focus
point of $g$, $G=[3,20]$ .[Use Geogebra?] Draw the line through F and G to find the point of intersection of this line with the X axis, $x=15$, which is the desired value for $x$. |