If $f:\mathbb{R}-\left\{\frac{2a}{3}\right\}\rightarrow \mathbb{R}$ is a function defined by
\[
f(x)=\frac{2x+a}{3x-2a}
\]
and $(f\circ f)(x)=x$ for all $x\in \mathbb{R}-\left\{\frac{2a}{3}\right\}$, then $f(3)=$}
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Whenever a question gives $f(f(x))=x$, immediately think of an involutory function. Compute the composition and compare coefficients to determine unknown parameters.