Complete the following.

$($a$)$ Find $f(x)$ for the indicated values of $x,$ if possible.

$($b$)$ Find the domain of $f.$

$f(x)=\sqrt[\phantom{2}]{x}+8$ for $x=-9,a+2$

$($a$)$ Evaluate $f(x)$ for $x=-9.$ Select the correct choice below and, if necessary, fill in the answer box to complete your choice.

A$.f(-9)=$

$($Type an exact answer, using radicals as needed. Simplify your answer.$)$

B$.$ The value of $f(-9)$ is undefined.

Evaluate $f(x)$ for $x=a+2$ and simplify your answer. Select the correct choice below and, if necessary, fill in the answer box to complete your ch

A$.f(a+2)=$

$($Type an exact answer, using radicals as needed. Type an expression using a as the variable. Simplify your answer.$)$

B$.$ The value of $f(a+2)$ is undefined for all values of $a.$