a,$(-12)-|13-x|=-21$
$⇒-|13-x|=-9$
$⇒|13-x|=9$
⇒\(\left[ \begin{array}{l}13-x=9\\13-x=-9\end{array} \right.\)
⇒\(\left[ \begin{array}{l}x=4\\x=-22\end{array} \right.\)
c,$x-(-25+x)=13-x$
$⇒x+25-x=13-x$
$⇒x=-12$
d,$(15-30)+x=x-(27-|-8|)$
$⇒-15+x=x-(27-8)$
$⇒0x=-4$ (vô lí)
⇒$x∈Ф$