\documentclass{article}

\usepackage{amsmath}

\begin{document}

\[
   F(x,y)=
   \underbrace
   {
      A x^2+2Bxy+Cy^2
   }_\text{2nd Order}
   +
   \underbrace
   {
      ax+by
   }_\text{1st Order}
   =c
\]

\end{document}