Answer
$f(18)=4$.
$f(3)=1$.
$f(2)=0$.
$f(-2)=$ Not a real number.
Work Step by Step
The given function is
$f(x)=\sqrt{x-2}$
Plug $x=18$ into the function.
$f(18)=\sqrt{18-2}$
Simplify.
$f(18)=\sqrt{16}$
$f(18)=\sqrt{4^2}$
$f(18)=4$
Plug $x=3$ into the function.
$f(3)=\sqrt{3-2}$
Simplify.
$f(3)=\sqrt{1}$
$f(3)=\sqrt{1^2}$
$f(3)=1$
Plug $x=2$ into the function.
$f(2)=\sqrt{2-2}$
Simplify.
$f(2)=\sqrt{0}$
$f(2)=0$
Plug $x=-2$ into the function.
$f(-2)=\sqrt{-2-2}$
Simplify.
$f(-2)=\sqrt{-4}$
Not a real number.