a) $x^2-x-20=0$
$⇔x^2-5x+4x-20=0$
$⇔(x-5)(x+4)=0$
$⇔\left[ \begin{array}{l}x-5=0\\x+4=0\end{array} \right.$
$⇔\left[ \begin{array}{l}x=5\\x=-4\end{array} \right.$
c) $|2x-3|-4x-9=0$
$⇔2x-3=4x+9$
$⇔\left[ \begin{array}{l}2x-3=4x+9\\2x-3=-4x-9\end{array} \right.$
$⇔\left[ \begin{array}{l}x=-6\\x=-1\end{array} \right.$
Chúc bạn học tốt !