It is true that $\displaystyle (6)^2=36$ and $\displaystyle (-6)^2 = 36$
BUT
$\displaystyle \sqrt {36} =6$ and $\displaystyle -\sqrt{36} = -6$.
The $\displaystyle \sqrt{}$ sign represents the positive square root.
The $\displaystyle -\sqrt{}$ sign represents the negative square root.
In other words, $\displaystyle \sqrt{anything}$ will never be negative.