If for some \( a \in \mathbb{N} \), \( f(f(f(a))) = 21 \), then \(\displaystyle \lim_{x \to a^-} \left\{ \frac{|x|^3}{a} - \left[ \frac{x}{a} \right] \right\} \), where \( [t] \) denotes the greatest integer less than or equal to \( t \), is equal to:
