It will always be negative, because negative times negative times negative is positive. for example, -1 x -1 x -1 = 1 x -1 = -1
So when finding cube root you find the number which can multiply itself three times to get the number.
Cube root of -8 is -2
because -2 x -2 x -2
=4 x -2
=-8
Thus all cube root of negative numbers are negative because negative times negative times negative is negative.