Lời giải:
a.
$(3x+2)^2-(x+3)(x+4)-8(x^2+1)=9x^2+12x+4-(x^2+7x+12)-(8x^2+8)$
$=9x^2+12x+4-x^2-7x-12-8x^2-8$
$=5x-16$
b.
$(4x-3)^2-(4x-3)(2x-1)-8x(x-2)+2x-6$
$=(4x-3)(4x-3-2x+1)-8x(x-2)+2x-6$
$=(4x-3)(2x-2)-8x(x-2)+2x-6$
$=8x^2-14x+6-8x^2+16x+2x-6$
$=4x$