Please explain to me how:

$\displaystyle \lim_{x\to\--\infty} \frac{3x}{4x-\sqrt{16x^2(1-\frac{3}{16x})}}$

becomes

$\displaystyle \lim_{x\to\--\infty} \frac{3x}{4x-(-4x)\sqrt{(1-\frac{3}{16x})}}$

Bringing 16x^{2}out from the radical becomes negative because x is approaching negative infinity? I don't quite understand why the other coefficients would not automatically become negative, or more importantly why is this one an exception??. Is there some vast concept at work here, or should I just devote this "rule" to route memorization?