$4)_{}$ $ax^2-5x^2-ax+5x+a-5_{}$
$=(ax^2-ax+a)-(5x^2-5x+5)_{}$
$=a.(x^2-x+1)-5.(x^2-x+1)_{}$
$=(x^2-x+1)(a-5)_{}$
$2)_{}$ $-4a-8b-12c_{}$
$=4.(-a-2b-3c)_{}$
$1)_{}$ $2x^2+4x-16_{}$
$=2.(x^2+2x-8)_{}$
$=2.(x^2+4x-2x-8)_{}$
$=2. [ x.(x+4)-2.(x+4)]_{}$
$=2.(x+4)(x-2)_{}$