TeX source:
\begin{array}{rclcl} (-7x+7)(7x-7)-31 &=& 21x^2+60 \cr -49x^2+49x+49x-49-31 &=& 21x^2+60 & \vert & -21x^2-60 \cr -70x^2+98x-140 &=& 0 & \vert & :(-70) \cr x^2-\dfrac{7}{5}x+2 &=& 0 \cr x_{1,2} &=& \dfrac{7}{10} \pm \sqrt{\dfrac{49}{100}-2} \cr x_{1,2} &=& \dfrac{7}{10} \pm \sqrt{-\dfrac{151}{100}} \end{array}