For the function

$f(x)=\frac{x^2-9}{x-3}$

$($a$)$ What is $f(3)?$

$($b$)$ What is the domain of $f(x)?.$

$($c$)$ Does $\underset{x3}{lim}\frac{x^2-9}{x-3}$ exist? If so$,$ what is it$?$

$($d$)$ Can we assign a value to $f(3)$ to make $f(x)$ continuous at $x=3?$