Let \( f : \mathbb{R} - \left\{ \dfrac{\alpha}{6} \right\} \to \mathbb{R} \) be defined by \( f(x) = \dfrac{5x + 3}{6x - \alpha} \). Then the value of \( \alpha \) for which \( (f \circ f)(x) = x \), for all \( x \in \mathbb{R} - \left\{ \dfrac{\alpha}{6} \right\} \), is:
