Incorrect. Write the radical as the quotient of two radicals and then rationalize the denominator.

$\displaystyle\sqrt{{\frac{{{12}{a}}}{{{3}{b}}}}}=\frac{\sqrt{{{12}{a}}}}{\sqrt{{{3}{b}}}}\cdot\frac{\sqrt{{{3}{b}}}}{\sqrt{{{3}{b}}}}\usepackage{cmbright}\Huge\$

Multiply the radicals on top and on bottom, then simplify.