4x2−36x+56

=4(x2−9x+14)

=4(x−2)(x−7)
VẬY : .....................

x^2 - 4x + 4 = 9 . [ x - 2 ]
x^2 - 4x + 4 = 9 . ( x − 2 )
x^2 - 4x + 4 = 9x - 18
x^2 - 4x + 4 - ( 9x - 18 ) = 0
x^2 - 4x + 4 - 9x + 18 = 0
x^2 - 4x + 22 - 9x = 0
x^2 - 13x + 22 = 0
x = 11
x = 2

phân tích 

4x^2 - 36x + 56
= (2x)^2 - 2.2x.9 + 9^2 - 25
= (2x - 9)^2 - 5^2
= (2x - 9 - 5)(2x - 9 + 5)
= (2x - 14)(2x - 4)
tìm x

x^2- 4x+4=9[x-2]^2
=> (x - 2)² - 9.(x - 2)² = 0
=> -8.(x - 2)² = 0
=> x - 2 = 0
=> x = 2
Vậy x = 2.