a, ( x - 1 ) ( x + 1 ) - ( x - 4 )² - x(x+7)
=x^2-1-x^2+8x-16-x^2-7x
=x-x^2-17
b)3x^3+x^2-5x+a=(3x^3+7x^2+9x)-(6x^2-14x-18)+(a+18)=x(3x^2+7x+9)-2(3x^2+7x+9)+(a+18)
=(x-2)(3x^2+7x+9)+(a+18)
vì (x-2)(3x^2+7x+9) chia hết cho (x-2) nên (a+18) chia hết cho (x-2) thì a+18=0=> a=-18