\documentclass[12pt]{article}
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\begin{document}
\begin{equation*}
  \begin{aligned}
   (a) \quad 
     G_{yr}(s) &= \frac{bk_ds^2+bk_ps+bk_i}{(1+bk_d)s^2+(a+bk_p)s+bk_i}, \\ 
   (b) \quad
     G_{yr}(s) &= \frac{bk_i}{(1+bk_d)s^2+(a+bk_p)s+bk_i}.
  \end{aligned}
\end{equation*}
\end{document}