\(\Leftrightarrow\left(x^2+8+9x\right)\left(x^2+8+6x\right)=28x^2\)
\(\Leftrightarrow\left(x^2+8\right)^2+15x\left(x^2+8\right)+54x^2-28x^2=0\)
\(\Leftrightarrow\left(x^2+8\right)^2+15x\left(x^2+8\right)+26x^2=0\)
\(\Leftrightarrow\left(x^2+13x+8\right)\left(x^2+2x+8\right)=0\)
hay \(x\in\left\{\dfrac{-13+\sqrt{137}}{2};\dfrac{-13-\sqrt{137}}{2}\right\}\)