Đáp án:
$\begin{array}{l}
a)3x - 7\sqrt x - 20\\
= 3x - 12\sqrt x + 5\sqrt x - 20\\
= 3\sqrt x \left( {\sqrt x - 4} \right) + 5\left( {\sqrt x - 4} \right)\\
= \left( {\sqrt x - 4} \right).\left( {3\sqrt x + 5} \right)\\
b){x^2} - x\sqrt x - 5x - \sqrt x - 6\\
= {x^2} + 2x\sqrt x - 3x\sqrt x - 6x + x + 2\sqrt x - 3\sqrt x - 6\\
= x\sqrt x \left( {x + 2} \right) - 3x\left( {\sqrt x + 2} \right)\\
+ \sqrt x \left( {\sqrt x + 2} \right) - 3\left( {\sqrt x + 2} \right)\\
= \left( {\sqrt x + 2} \right)\left( {x\sqrt x - 3x + \sqrt x - 3} \right)\\
= \left( {\sqrt x + 2} \right)\left( {\sqrt x - 3} \right)\left( {x + 1} \right)
\end{array}$
