x^3+3x-4
=x^3 + (x^2 - x^2) + (4 x - x) - 4
=x^3 + x^2 + 4 x - x^2 - x - 4
= (x^2 + x + 4) x - (x^2 + x + 4)
= (x^2 + x + 4)(x -1)
4 x^2 + 2 x - y - y^2
=4 x^2 - y^2 +(2 x - y)
=(2 x + y)(2 x - y)+(2 x - y)
=(2 x + y + 1)(2 x - y)
x^3 - y^3 - x^2 y + x y^2
=(x - y) (x^2 + x y + y^2) - x (x - y) y
=(x - y) (x^2 + y^2)