Answer:
Step-by-step explanation:
[tex]\displaystyle \Large \boldsymbol{} x^2+10x+9=0 \\\\ x^2+\underbrace{x+9x}_{10x}+9=0 \\\\x^2+x+\underline9x+\underline9=0 \\\\x(x+1)+9(x+1)=(x+9)(x+1)[/tex]
