Something like this?
The \mkern2mu
directive inside the argument of \mathllap
serves to places the \times
symbol right below the gap between the two horizontal braces. (A larger value for the argument of \mathllap
would push the \times
symbol further to the left.) Likewise, \mathrlap{\cramped{\,=\,2^{12}}}
serves to place =2^{12}
to the right of the main argument of the second \underbrace
directive.

\documentclass{article}
\usepackage{mathtools} % for '\mathllap', '\mathrlap', and '\cramped' macros
\begin{document}
\[
\underbrace{4}_{2^2}
\underbrace{\mathrm{KB}}%
_{\mathllap{\times\mkern2mu}
2^{10} % <-- main argument of '\underbrace'
\mathrlap{\cramped{\,=\,2^{12}}}}
\]
\end{document}