Answer
a) $f(28)= 6$
b) $x= 113$
Work Step by Step
Given $$f(x)=\sqrt{x+8}.\tag{1}$$ a) Set $x= 28$ in equation $(1)$ and determine $f(28)$:
\begin{equation}
\begin{aligned}
f(28)&=\sqrt{28+8} \\
&=\sqrt{36}\\
&=6.
\end{aligned}
\end{equation} b) Set $f(x)=11$ in equation $(1)$ and solve for $x$:
\begin{equation}
\begin{aligned}
f(x) & =11 \\
\sqrt{x+8}& =11 \\
\left(\sqrt{x+8}\right)^2& = \left(11\right)^2\\
x+8&=121\\
x&= 121-8\\
x&= 113.
\end{aligned}
\end{equation}
