the idea being, you multiply top and bottom by a value that will raise the radicand in the denominator, to the same as the root, thus coming out of the root, so, let's do so
[tex]\bf \cfrac{3}{\sqrt{12x}}\cdot \cfrac{\sqrt{12x}}{\sqrt{12x}}\implies \cfrac{3\sqrt{12x}}{\sqrt{(12x)^2}}\implies \cfrac{3\sqrt{12x}}{12x}\implies \cfrac{\sqrt{12x}}{4x}[/tex]
