\[\boxed{\text{314\ (314).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ y = \sqrt{12x - 3x^{2}}\ \]
\[12x - 3x^{2} \geq 0\]
\[x^{2} - 4x \leq 0\]
\[x(x - 4) \leq 0\]
\[x \in \lbrack 0;4\rbrack.\]
\[\textbf{б)}\ y = \frac{1}{\sqrt{2x^{2} - 12x + 18}}\]
\[2x^{2} - 12x + 18 > 0\]
\[x^{2} - 6x + 9 > 0\]
\[(x - 3)^{2} > 0\]
\[x \in ( - \infty;3) \cup (3;\ + \infty).\]