Given the function defined in the table below, find the average rate of change, in simplest form, of the function over the interval [tex]$18 \leq x \leq 24$[/tex].
\begin{tabular}{|c|c|}
\hline
[tex]$x$[/tex] & [tex]$f(x)$[/tex] \\
\hline
12 & 41 \\
\hline
18 & 40 \\
\hline
24 & 39 \\
\hline
30 & 38 \\
\hline
\end{tabular}