Answer:
[tex]\large\boxed{x=-3\pm\sqrt{21}}[/tex]
Step-by-step explanation:
[tex]x^2+6x=12\\\\x^2+2(x)(3)=12\qquad\text{add}\ 3^2\ \text{to both sides}\\\\x^2+2(x)(3)+3^2=12+3^2\qquad\text{use}\ (a+b)^2=a^2+2ab+b^2\\\\(x+3)^2=12+9\\\\(x+3)^2=21\iff x+3\pm\sqrt{21}\qquad\text{subtract 3 from both sides}\\\\x=-3\pm\sqrt{21}[/tex]
