Assuming i is the conventional usage, the square root of -1, multiply numerator and denominator by i. This gives you 6i/-3, or -2i. You could be a wise guy and call it 2i^3 but you would probably lose points for that.
When rationalizing any expression, look at the denominator to see what you can do to get rid of ugly things like i, roots and wacky exponents. i is almost always best dealt with by multiplying by i to make it real (-1).
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