Answer
$x= 7.5$
Work Step by Step
The graph shows that when $x= 7.5$, $f(x)= 1$. Alternatively, we can use the values of $m$ and $b$ to write the equation of the line and use it to find $x$.
The line is $y= -\frac{2}{5}x+4$. Lets check our answer instead. $$f(7.5) = -\frac{2}{5}\cdot 7.5+4 = 1.8\ \checkmark$$
