Đáp án:
a,$3.(-5)²+2. (-5)-20$
=$(-5).(-5.3+2-4)$
=$(-5).(-17)$
$=85$
b,$5-(10-x)=-7$
⇔$5-10+x=-7$
⇔$x=-7+10-5$
⇔$x=-2$
c,$|x-3|=9$
⇔\(\left[ \begin{array}{l}x-3=9\\x-3=-9\end{array} \right.\)
⇔\(\left[ \begin{array}{l}x=12\\x=-6\end{array} \right.\)