Answer:
The answer is option 1.
Step-by-step explanation:
Firstly, you have to elaborate the expression :
[tex] {x}^{2} + 10x + 21[/tex]
[tex] = {x}^{2} + 3x + 7x + 21[/tex]
Next you have to take out the common factors and factorize it :
[tex] {x}^{2} + 3x + 7x + 21[/tex]
[tex] = x(x + 3) + 7(x + 3)[/tex]
[tex] = (x + 7)(x + 3)[/tex]
