For some constants \(a\) and \(b\), let
\[f(x) = \left\{
\begin{array}{cl}
ax + b & \text{if } x < 2, \\
8 - 3x & \text{if } x \ge 2.
\end{array}
\right.\]
The function \(f\) has the property that \(f(f(x)) = x\) for all \(x.\) What is \(a + b?\) Let's think step by step and output the final answer within \boxed{}.
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