Let $f(x) = %0.$ The natural domain of this function is the set of all real numbers, as $f(x)$ makes sense for any real number $x$. Then, to find, say, $f(2)$, you just substitute $2$ for $x$ in the formula:

$f(x) $	$= %0$	Given formula.
$f(2) $	$= %9 \qquad$	Substitute $(2)$ for $x$; good idea to use parentheses.
	$= %10$
	$= %11$

Similarly, we can calculate $f(-3)$ as follows:

$f(x) $	$= %0$	Given formula.
$f(-3) $	$= %12 \qquad$	Substitute $(-3)$ for $x$; important to use parentheses for negative values of $x$.
	$= %13$
	$= %14$

Some for you: