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:
Let $f(x) = %0$. Then
 
$f(%1)$ = BOX
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$f(%2)$ = BOX
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$f(%3)$ = BOX
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Let $f(x) = %0$. Then
 
$f(%4)$ = BOX* ACTIVEMATH
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$f(-%4)$ = BOX* ACTIVEMATH
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$-f(%4)$ = BOX* ACTIVEMATH
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Let $f(x) = %0$. Then
 
$f(%4+%5)$ = BOX* ACTIVEMATH
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Let $f(x) = %0$. Then
 
$f(%4-%5)$ = BOX* ACTIVEMATH
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