My school book says that for $\sqrt a$ $*$ $\sqrt b$ $=$ $\sqrt ab$ to be an valid operation atleast one of $a$ or $b$ should to be non-negative.They support this argument using this example
$6$ $=$ $\sqrt 36$ $=$ $\sqrt{ (-4)*(-9)}$ $=$ $\sqrt{ -4}$$\sqrt {-9}$$=$ $2i*3i$ $=$ $6i^2$$=$ $-6$ which is false
but as we know $\sqrt 36$ have two root $\+- 6$ so probably their argument is wrong isn't it?