[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}\\\\
-------------------------------\\\\
f(x)= \cfrac{1}{x} \qquad
\begin{cases}
x_1=2\\
x_2=b
\end{cases}\implies \cfrac{f(b)-f(2)}{b-2}=-\cfrac{1}{10}
\\\\\\
\cfrac{\frac{1}{b}-\frac{1}{2}}{b-2}=-\cfrac{1}{10}\implies
\cfrac{\frac{2-b}{2b}}{b-2}=-\cfrac{1}{10}\implies \cfrac{2-b}{2b}=-\cfrac{b-2}{10}
\\\\\\
[/tex]
[tex]\bf \cfrac{2-b}{2b}=\cfrac{2-b}{10}\implies 20-10b=4b-2b^2\impliedby cross-multiplying
\\\\\\
2b^2-4b-10b+20=0\implies 2b^2-14b+20=0
\\\\\\
b^2-7b+10=0\implies
(b-2)(b-5)=0\implies b=
\begin{cases}
\boxed{5}\\
2
\end{cases}[/tex]
