a, $A=a(2b-c)-b(a+c)-a(c+b)$
$A=2ab-2ac-ab-bc-ac-ab$
$A=-3ac-bc$
b, $B=(a+3b)(c-d)-(3a-d)(b+c)-2c(b-a)+2b(a+b)$ (Câu này mình không chắc)
$B=ac-ad+3bc-3bd-(3ab+3ac-bd-cd)-2bc+2ac+2ab+2b^2$
$B=ac-ad+3bc-3bd-3ab-3ac+bd+cd-2bc+2ac+2ab+2b^2$
$B=-ad+bc-2bd-ab+2b^2$