2x^2+3xy-9y^2+14x-3y+20
=(x+3y)(2x-3y)+4(x+3y)+10x-15y+20
=(x+3y)(2x-3y+4)+5(2x-3y+4)
=(x+3y+5)(2x-3y+4)
x^4+5x^3+15x-9
=x^4-9+5x(x²+3)
=(x²+3)(x²-3)+5x(x²+3)
=(x²+3)(x²+5x-3)