\documentclass{article}
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\usepackage{amssymb}
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\begin{document}
\begin{equation}
  y=\left\{\begin{array}{c}
      y_{\rm break}(\frac{x}{x_{\rm break}})^{\gamma_1} (x<x_{\rm break})\\
      y_{\rm break}(\frac{x}{x_{\rm break}})^{\gamma_2} (x>=x_{\rm break})
      \end{array}
      \right .\notag
\end{equation}
\end{document}