a) $3\sqrt{x} = \sqrt{12} \qquad (x \geq 0)$
$\Rightarrow 9x = 12$
$\Rightarrow x = \dfrac{12}{9} = \dfrac{4}{3}$ (nhận)
Vậy $x = \dfrac{4}{3}$
b) $\sqrt{2x-1} = 2 \qquad \left(x \geq \dfrac{1}{2}\right)$
$\Rightarrow 2x - 1 = 4$
$\Leftrightarrow 2x = 5$
$\Leftrightarrow x = \dfrac{5}{2}$ (nhận)
Vậy $x = \dfrac{5}{2}$