sqrt question

[Claudio L.]

Both answers are correct!
Every power in the form a^(1/n) has 'n' answers in the complex plane. sqrt is the particular case n=2, so it has 2 answers:
sqrt(-4) = +2i and -2i
sqrt(-9) = +3i and -3i
doing all 4 possible combinations for the product you arrive at the 2 possible answers: +6 and -6.

sqrt( (-4)*(-9) ) = sqrt(36) = +6 and -6

By doing the operations in one order or the other, you are simply choosing (or accidentally falling into) one branch or the other, nothing wrong with that, but don't ever forget that there's 2 sides of that coin.