Câu trả lời:

a, x² + 7x + 10

= x² + 2x + 5x + 10

= x(x + 2) + 5(x + 2)

= (x + 2)(x + 5)

b, 3x² + 12x + 9

= 3x² + 9x + 3x + 9

= 3x(x + 3) + 3(x + 3)

= (3x + 3)(x + 3)