#### Answer

$-x\sqrt {5x}$

#### Work Step by Step

$\sqrt {125x^{3}}-3\sqrt {20x^{3}}$
=$\sqrt {25\times5\times x^{2}\times x}-3\sqrt {4\times5\times x^{2}\times x}$
=$5x\sqrt {5\times x}-3(2x)\sqrt {5\times x}$
=$5x\sqrt {5x}-6x\sqrt {5x}$
=$(5x-6x)\sqrt {5x}$
=$-x\sqrt {5x}$