a)
5x^4-20=5(x^4-4)=5(x^2-2)(x^2+2)=5(x-2)(x+2)(x^2+2)
b)
-4x^4+26x^3-12x^2=-2x^2(2x^2-13x+6)=-2x^2(2x^2-x-12x+6)=
-2x^2[x(2x-1)-6(2x-1)]=-2x^2(2x-1)(x-6)
c)
2x^3+5x^2+6x+15=2x^3+6x+5x^2+15=2x(x^2+3)+5(x^2+3)=
2x(x^2+3)(2x+5)
d)
x^5-2x^4+7x^3+8x^2-16x+56=x^5+8x^2-2x^4-16x+7x^3+56=
x^2(x^3+8)-2x(x^3+8)+7(x^3+8)=(x^3+2^3)(x^2-2x+7)=
(x+2)(x^2+2x+4)(x^2-2x+7)