The data in the table is inversely proportional. Find the value of $a$.
\multicolumn{4}{|c|}{\begin{tabular}[t]{|l|l|l|l|}
X4 9 12
F(x)18 8 a
} \\
\hline
\end{tabular}
\multicolumn{4}{|c|}{\begin{tabular}[t]{|l|l|l|l|} X | 4 | 9 | 12 |
F(x) | 18 | 8 | a |