Answer
$m= b^5-b$
Work Step by Step
First find the coordinates of the two points on the graph.
\begin{equation}
\begin{aligned}
P(x_1, y_1)&=(1, 4b)\\
Q(x_2, y_2)&=(5, 4b^5)\\
m&= \frac{y_2-y_1}{x_2-x_1}\\
&= \frac{4b^5-4b}{5-1}\\
&= b^5-b\\
y&= mx-mx_1+y_1\\
&= (b^5-b)x-(b^5-b)+4b\\
y&= (b^5-b)x-b^5+5b\\
\end{aligned}
\end{equation}
The slope of the line segment is $m= b^5-b$.