Đáp án:

 3)

x^2+2x-y^2+1

=x^2+2x+1-y^2

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

=(x+1+y)(x+1-y)

6)

x^2+4x-y^2+4

=x^2+4x+4-y^2

=(x+2)^2-y^2

=(x+2+y)(x+2-y)

9)

x^2+8x+16-y^2

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

=(x+4-y)(x+4+y)

12)

x^2-6x-4y^2+9

=x^2-6x+9-4y^2

=(x-3)^2-(2y)^2

=(x-3-2y)(x-3+2y)

15)

x^2-y^2+14x+49

=x^2+14x+49-y^2

=(x+7)^2-y^2

=(x+7-y)(x+7+y)

18)

x^2+10x-16y^2+25

=x^2+10x+25-16y^2

=(x+5)^2-(4y)^2

=(x+5-4y)(x+5+4y)

21)

16y^2-4x^2-12x-9

=16y^2-(4x^2+12x+9)

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

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

@Sad2006