PT $\Leftrightarrow (6x+7)^2 (6x+8)(6x+6)=6.12 =72$
Đặt $6x+7 = t$
PT $\Leftrightarrow t^2 (t+1)(t-1)-72=0$
$\Leftrightarrow t^2 (t^2-1)-72=0$
$\Leftrightarrow t^4 -t^2 -72=0$ đặt $t^2 = u \ge 0$
PT$\Leftrightarrow u^2-u-72=0$
$\Leftrightarrow (u+8)(u-9)=0$
+ $u+8 = 0$ vô nghiệm (d0 $u \ge 0$)
+ $u= 9 = t^2 \Leftrightarrow t =\pm 3$
+ $t=3 =6x+7 \Rightarrow x= -\dfrac{2}{3}$
+ $t=-3 =6x+7 \Rightarrow x= -\dfrac{5}{3}$