[tex](f+g)(x)+f(x)+g(x)\\\\f(x)=x^2-2x\\\\g(x)=6x+4\\\\\text{Substitute:}\\\\(f+g)(x)=(x^2-2x)+(6x+4)=x^2-2x+6x+4\\\\\text{combine like terms}\\\\(f+g)(x)=x^2+(-2x+6x)+4=x^2+4x+4\\\\(f+g)(x)=0\Rightarrow x^2+4x+4=0\\\\x^2+2x+2x+4=0\\\\x(x+2)+2(x+2)=0\\\\(x+2)(x+2)+0\\\\(x+2)^2=0\iff x+2=0\qquad\text{subtract 2 from both sides}\\\\x=-2[/tex]
