When \(a \ne 0\), there are two solutions to \(ax^2 + bx + c = 0\) and they are \[x = {-b \pm \sqrt{b^2-4ac} \over 2a}.\] I can also use LaTeX aligned environment. \begin{aligned} \delta_{k}(x) &=-\frac{1}{2 \sigma_{k}^{2}}\left(x-\mu_{k}\right)^{2}+\log \pi_{k} \\ &=-\frac{1}{2 \sigma_{k}^{2}} x^{2}+\frac{x \mu_{k}}{\sigma_{k}^{2}}-\frac{\mu_{k}^{2}}{2 \sigma_{k}^{2}}+\log \pi_{k} \end{aligned}