How would I underline an equation with a curly brace and an explanation?

Question

How would I underline an equation with a curly brace and an explanation?, like I've pictured below?

I've tried to use the amsmath package, but the \underline directive does not produce the brace and creates ugly space between the A, B and =.

Answer 1

\underline is not \underbrace. Use

 \usepackage{amsmath}
  ..........
 \begin{document}
[ \underbrace{A=B}_{\text{Fermat}} ]
  ...........
\end{document}