Answer:
-6
Step-by-step explanation:
A suitable calculator can find this for you directly.
Odd root of a negative number
-1 raised to an odd power is -1. That means an odd-index root of a negative number will be the opposite of the corresponding root of its absolute value:
∛(-216) = -∛216 = -6
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Additional comment
Many calculators and spreadsheets do not respond well to requests for roots of negative numbers. A calculator with built-in support for complex numbers is the best choice for this sort of problem.
The second attachment shows a calculator capable of finding the complex roots as well as the real one.
