The table below shows the number of laps Carly ran around the track each day. \begin{tabular}{|c|c|}
\hline
Day & Number of Laps \\
\hline
2 & 6 \\
3 & 9 \\
4 & 12 \\
5 & 15 \\
\hline
\end{tabular} Which rule describes the pattern shown by the number of laps Carly ran?