\documentclass{article} \usepackage{amsmath} \begin{document} \begin{equation} \begin{aligned} \min_{w,b,\xi} \quad & \frac{1}{2}w^{t}w+C\sum_{i=1}^{N}{\xi_{i}}\\ \textrm{s.t.} \quad & y_{i}(w\phi(x_{i}+b))+\xi_{i}-1\\ &\xi\geq0 \\ \end{aligned} \end{equation} \end{document}