Consider the function

a$)$ Simplify the expression for $f(x)$ when

$\frac{x^2-1x-12}{x+3}=$

$$

b$)$ Compute the following limit$.$

$\underset{x-3^{-}}{lim}f(x)=$

$$

c$)$ Compute the following limit$.$

$\underset{x-3^{+}}{lim}f(x)=$

d$)$ Find the value of a such that $f$ is continuous at $x=-3.$

Answer: $a=$