The square root of a (necessarily) positive number is always positive: $\sqrt {a^2} = a$ if $a \ge 0$ $\sqrt {a^2} = -a$ if $a \le 0$ $\sqrt {a^2} = \vert a \vert $ (absolute value of a !)

Go !

$\sqrt{(-3)^2} =$ $\vert-3\vert = -(-3) = 3$
$\sqrt{-3}$
$\sqrt{a^4}=$