Answer :
a. [tex]f[/tex] has an average value on [5, 11] of
[tex]f_{\rm ave}=\displaystyle\frac1{11-5}\int_5^{11}(x-7)^2\,\mathrm dx=\frac{(x-7)^3}{18}\bigg|_5^{11}=\frac{4^3-(-2)^3}{18}=4[/tex]
b. The mean value theorem guarantees the existence of [tex]c\in(5,11)[/tex] such that [tex]f(c)=f_{\rm ave}[/tex]. This happens for
[tex](c-7)^2=4\implies c-7=\pm2\implies c=9\text{ or }c=5[/tex]