I assumed that since ac⋅bc=(ab)c, then something like √−4⋅√−9 would be √−4⋅−9=√36=±6 but according to Wolfram Alpha, it's −6?
Answer
The property ac⋅bc=(ab)c that you mention only holds for integer exponents and nonzero bases. Since √−4=(−4)1/2, you cannot use this property here.
Instead, use imaginary numbers to evaluate your expression:
√−4⋅√−9=(2i)(3i)=6i2=−6
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