x(x - 7) - 3x + 21 = 0

<=> x(x - 7) - 3(x - 7) = 0

<=> (x - 3)(x - 7) = 0

<=> \(\left[{}\begin{matrix}x-3=0\\x-7=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=3\\x=7\end{matrix}\right.\)

4x²z + 8xyz + 4y²z

= z(4x² + 8xy + 4y²)

= \(z\left[\left(2x\right)^2+2.2x.2y+\left(2y\right)^2\right]\)

= z(2x + 2y)²

anh giỏi quá

a. (3x - 1)² - 16

= (3x - 1)² - 4²

= (3x - 1 - 4)(3x - 1 + 4)

= (3x - 5)(3x + 3)

= 3(3x - 5)(x + 1)

b. (5x - 4)² - 49x²

= (5x - 4)² - (7x)²

= (5x - 4 - 7x)(5x - 4 + 7x)

= \(\left(-2x-4\right)\left(2x-4\right)\)

= 2(-x - 2) . 2(x - 2)

= 4(-x - 2)(x - 2)

c. (2x + 5)² - (x - 9)²

= (2x + 5 - x + 9)(2x + 5 + x - 9)

= (x + 14)(3x - 4)

Bn chia câu hỏi ra cho mn dễ làm nhé