\documentclass[fleqn,leqno]{article}
\usepackage{hypertlabook}
\pdftitle{Answer}
\begin{popup}
\subsection*{Answer}

$ \{ f \in [\{"a",\, "b"\} -> A \cup B] :
        (f["a"] \in A) \, /\ \, (f["b"] \in B) \}
$
\end{popup}
\makepopup