TeX source:

\begin{aligned}x &=\dfrac{-b \pm \sqrt{b^{2}-4 a c}}{2 a} \\&=\dfrac{-(1) \pm \sqrt{(1)^{2}-(4) \cdot(1) \cdot(2)}}{2 \cdot 1} \\&=\dfrac{-1 \pm \sqrt{1-8}}{2} \\&=\dfrac{-1 \pm \sqrt{-7}}{2} \\&=\dfrac{-1 \pm i \sqrt{7}}{2}\end{aligned}