another way to look at it is:
\(\displaystyle \sqrt{x} = x^{1/2}\), so
\(\displaystyle (a\sqrt{x})(b\sqrt{x}) = (ab)(\sqrt{x}\sqrt{x})\)
\(\displaystyle = (ab)(x^{1/2})(x^{1/2}) = (ab)(x^{(1/2 + 1/2)}) = (ab)x^1 = (ab)x\)
using the familar "laws of exponents".
for the time being, it's best to restrict such use (of exponent laws) to "rational exponents" and "non-negative x", for reasons which are too far off-topic to go in to.