TeX source:
\begin{array}{rclll} \mathbb{D} &=& \mathbb{R} \cr\cr f'(x) &=& -5x+10 \cr \cr 0 &=& -5x+10 & \vert & +5x \cr 5x &=& 10 & \vert & :5 \cr x &=& 2 \end{array}