Question 65678
3 sqrt -8/27 
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You are right in saying "this is not a Real Number".
It is not part of the Real Number System.
It is a number in the Complex Number System.
In that system it equals:
3[(2/3)sqrt(-2/3)]
=2isqrt(2/3)
Note: "i" is the square root of negative one.
=(2/3)isqrt6
Cheers,
Stan H.