a) 4x^2-9
= (2x)^2 - 3^2
=  (2x+3). (2x-3)
b) x^3-9x^2 +27x-35=0
⇔x^3 - 3. x^2 . 3 + 3. x. 3^2 - 3^3 -8 =0
⇔(x-3)^3 = 8
⇔(x-3)^3 = 2^3
⇔x-3 = 2
⇔x= 5
Vậy x=5 
c) x^2-6x-7
=x^2 -7x + x -7
=x(x-7) + (x-7) .1
=(x-7)(x+1)
d) x^2+4x+3
=x^2 + x+3x+3
=x(x+1) + 3(x+1)
=(x+1)(x+3)