6) $\Rightarrow x^2-9+4x^2+12x=0$
$\Rightarrow 5x^2+12x-9=0$
$\Rightarrow 5x^2+15x-3x-9=0$
$5x(x+3)-3(x+3)=0$
$(5x-3)(x+3)=0$
$\Rightarrow \left[\begin{array}{l} x=\dfrac{3}{5} \\ x=-3 \end{array} \right .$
7) $\Rightarrow x(x+1)-2(x+1)=0$
$(x+1)(x-2)=0$
$\Rightarrow \left[\begin{array}{l} x=-1\\ x=2 \end{array} \right .$
8) $\Rightarrow x(x-1)+2(x-1)=0$
$(x-1)(x+2)=0$
$\Rightarrow \left[\begin{array}{l} x=1\\ x=-2 \end{array} \right .$
9) $x(3x-2)+3(3x-2)=0$
$(x+3)(3x-2)=0$
$\Rightarrow \left[\begin{array}{l} x=-3\\ x=\dfrac{2}{3} \end{array} \right .$
10) $x(x-1)+3(x-1)=0$
$\Rightarrow (x-1)(x+3)=0$
$\Rightarrow \left[\begin{array}{l} x=1\\ x=-3 \end{array} \right .$
