\[y = \sqrt{4x - 9x^{2}}\ \]
\[4x - 9x^{2} \geq 0\]
\[9x^{2} - 4x \leq 0\]
\[9x\left( x - \frac{4}{9} \right) \leq 0\]
\[Ответ:x \in \left\lbrack 0;\frac{4}{9} \right\rbrack.\]