You may want to review simplifying roots first.

As you know, you can't take the square root of a negative number.
But mathematicians (and, by “mathematicians”, I mainly
mean Gerolamo Cardano, a professional gambler who lived in what's
now Italy in the 1500s) have noticed that it's often useful to
essentially pretend you can. That is, we make up a number
called *i* with the property that *i*^{2} =
−1. Now, of course, √−1 = *i*.

If this bothers you, bear in mind that this is really no
different from √2. In the old days, the only numbers that
were thought of as “real” were rational numbers. But
it was useful to invent a number whose square was 2, so people
did. Similarly, it's useful (in a number of fields) to invent a
number whose square is −1, so we do that. Of course, unlike
√2, *i* doesn't appear anywhere on the number line.
When we want to refer specifically to numbers on the number line,
we'll call them *real* numbers.

Now, we can take the square root of any negative number. For
example, √−12 = √−1 * √12 = *i*
* 2√3 = 2√3**i*.