Решите пожалуйста Дам 20 баллов

$1) 4n-nc-4+c= \\ n(4-c)-(4-c)=(n-1)(4-c)$
В
$2) 2ax-bx+10ay-5by=\\ 2a(x+5y)-b(x+5y) =\\ =(2a-b)(x+5y)$
$3) 17.2*8.1-17.2*7.6+23.8*5.1-\\ -23.8*4.6=\\ 17.2(8.1-7.6) +23.8(5.1-4.6)=\\ =17.2*0.5+23.8*0.5=\\ =(17.2+23.8)=\\ =1/2*41=20.5$
$4) x^2*(x-9)+(x-9)=\\ =(x-9)(x^2+1)$
при $x=19; (19-9)(19^2+1)=\\10((20-1)^2+1)=\\=10(400-40+1+1)=\\=10*362=3620$
$5) (x+3)(x-15)=0$
[ $x=-3$
[ $x=15$
$6) x^2-5x+4=x^2-x-4x+4=\\x(x-1)-4(x-1)=(x-1)(x-4)$

$7) y^4-3y^3+y-3=0\\ y^3(y-3)+(y-3)=0\\ (y-3)(y^3+1)=0$
[ $y=3$
[ $y=-1$