TeX source:
\begin{array}{rclcl} 80+(11x-6)(11x+6)+4x^2 &=& 6\left(\dfrac{100}{3}x-6\right) \cr 80+121x^2-36+4x^2 &=& 200x-36 & \vert & +36 \cr 80+125x^2 &=& 200x & \vert & -125x^2-80 \cr 0 &=& -125x^2+200x-80 & \vert & :(-125) \cr 0 &=& x^2-\dfrac{8}{5}x+\dfrac{16}{25} \cr\cr x_{1,2} &=& \dfrac{4}{5} \pm \sqrt{\dfrac{16}{25}-\dfrac{16}{25}} \cr\cr x_{1,2} &=& \dfrac{4}{5} \cr\cr \mathbb{L} &=& \left\{\dfrac{4}{5}\right\}\end{array}