How to find two square roots of a number?

In fact for any number $a$ (in $\mathbb{R}$ or $\mathbb{C}$), if $r$ is a square root of $a$ then $- r$ is also a square root of $a$.
If $a \in \mathbb{R}$ and $a > 0$ then $a$ has two real square roots - the positive square root that we represent by the symbols $\sqrt{a}$ and the negative square root, which is $- \sqrt{a}$.
If $a \in \mathbb{R}$ and $a = 0$ then $a$ has one (repeated) square root, viz $0$.
If $a \in \mathbb{R}$ and $a < 0$ then $a$ has two pure imaginary square roots, $\sqrt{- a} \cdot i$ and $- \sqrt{- a} \cdot i$