new row latex code example

Example 1: latex add empty line

\hfill \break

Example 2: latex addition row

\documentclass{article}
\begin{document}
\begin{tabular}{cccc}
  & 1 & 2 & 3 \\
+ &   & 3 & 4 \\
\hline
  & 1 & 5 & 7 \\
\end{tabular}
\end{document}