\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
      T_{a}(x) = 25 + \left(T_{F}-25
      \right) \left(\frac{T_{b}-25}{T_{F}-25}\right)^{(x-1)^2}
\end{equation*}
\end{document}