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If the cubic root is in the denominator, then you could write 1/cubr(-1), or 1/(-1)^(1/3), or (-1)^(-1/3), cubr((-1)^(-1)) The index 3 on the radical must be positive. It could be a typo error in you homework assignment.
is your problem -cubr(-1)? that could be written as -(-1)^(1/3). the answer would be 1.