check the picture below.
[tex]\bf slope = m = \cfrac{rise}{run} \implies
\cfrac{ f(x_2) - f(x_1)}{ x_2 - x_1}\impliedby
\begin{array}{llll}
average~rate\\
of~change
\end{array}\\\\
-------------------------------\\\\
\begin{cases}
x_1=\frac{3\pi }{2}\\
x_2=2\pi
\end{cases}\implies \cfrac{f(2\pi )-f\left( \frac{3\pi }{2} \right)}{2\pi -\frac{3\pi }{2} }\implies \cfrac{3-1}{\frac{4\pi -3\pi }{2}}\implies \cfrac{2}{\quad \frac{\pi}{2}\quad }\implies \cfrac{4}{\pi }[/tex]
