c) $a\sqrt{\dfrac{13}{a}}$ (với $a>0$)
$=a.\dfrac{\sqrt{13}}{\sqrt{a}}$
$=\sqrt{a}.\sqrt{13}$
$=\sqrt{13a}$
d) $\dfrac{3}{a}.\sqrt{\dfrac{-6a}{5}}$ (với $a<0$)
$=\dfrac{3}{a}.\dfrac{\sqrt{-6a}}{\sqrt{5}}$
$=\dfrac{3\sqrt{6}}{\sqrt{-5a}}$
$=-\dfrac{3\sqrt{-30a}}{5a}$