#### Answer

$\frac{x\sqrt {2x}}{7y^{2}}$

#### Work Step by Step

$\sqrt \frac{2x^{3}}{49y^{4}}$
=$\sqrt \frac{2\times x^{2}\times x}{7^{2}\times y^{4}}$
=$\frac{\sqrt {2x}\times\sqrt {x^{2}}}{\sqrt {7^{2}}\times \sqrt {y^{4}}}$
=$\frac{x\sqrt {2x}}{7\times y^{2}}$
=$\frac{x\sqrt {2x}}{7y^{2}}$