#### Answer

$\frac{7x\sqrt {7}}{10}$

#### Work Step by Step

$\frac{3x\sqrt {7}}{5}+\sqrt \frac{7x^2}{100}$
$\frac{3x\sqrt {7}}{5}+\frac{\sqrt 7\sqrt {x^2}}{\sqrt {100}}$ Simplify.
$\frac{3x\sqrt {7}}{5}+\frac{x\sqrt {7}}{10}$ The LCD is 10.
$\frac{6x\sqrt {7}}{10}+\frac{x\sqrt {7}}{10}$
$\frac{7x\sqrt {7}}{10}$ Add.