When you ask for the square root of a positive number a : √a you ask to find a number c such that c × c = a
For example, take a = 4. Then as you know, √4 = √(2×2) because 4 = 2×2
And so you finally got this number c, such that c × c = 4 it is c = 2
Now the cubic root of a number a is the same principle, the only difference is we are looking for a (positive) number c, such that c × c × c = a, this number c will be the cubic root of ∛a
But we already know that 8 = 2 × 2 × 2
So we found that number c, such that c × c × c = 8, it is c = 2
Conclusion : -∛8 = - ∛(2×2×2) = - 2
Good Luck !