Check the picture below.
[tex](x+6)^2=x^2+12^2\implies (x+6)(x+6)=x^2+144 \\\\\\ \stackrel{F~O~I~L}{x^2+12x+36}=x^2+144\implies 12x+36=144\implies 12x=108 \\\\\\ x=\cfrac{108}{12}\implies x = 9[/tex]
