I1.2 Given that $f(x)=-x^{2}+10 x+9$, and $2 \leq x \leq \frac{a}{9}$. If $b$ is the difference of the maximum and minimum values of $f$, find the value of $b$.
$\begin{array}{l}f(x)=-x^{2}+10 x+9=-(x-5)^{2}+34 \text { for } 2 \leq x \leq 8 \\ \text { Maximum }=f(5)=34 ; \text { minimum }=f(2)=f(8)=25 \\ b=34-25=9\end{array}$