TeX source:
\begin{array}{rclcl} 50\left(1-\dfrac{1}{5}x\right) &=& (x-5)^2 \cr 50-10x &=& x^2-10x+25 &\vert& +10x-25 \cr 25 &=& x^2 &\vert& \pm \sqrt{} \cr x &=& \pm \sqrt{25} \cr\cr x_1 &=& 5 \cr x_2 &=& -5 \cr \cr \mathbb{L} &=& \{-5; 5\} \end{array}