There is no real answer, YOu cannot find the squareroot of a negative number but if you know about complex numbers there is an imaginary answer.
x2=−1x=±i
The +- symbol means that the number can be both positive and negative. So, to elaborate on Meoldy's answer, x = -i and x = +i. You use it whenever you take the square root of a number, as both (-x)^2 and (x)^2 give the same result, without exception.
You'll see this in the quadratic formula: x=−b±√b2−4ac2a
That just means there are two answers, one where the +- sign is positive and one where it's negative.