Let f(x)={ x-1, x is even, 2 x, x is odd, x ∈ N.. If for some a ∈ N, f(f(f( a )))=21, then lim_x → a^- {(|x|^3)/a-[x/a]}, where [t] denotes the…

Let $\displaystyle f(x)=\left\{\begin{array}{l}x-1, x \text { is even, } \\ 2 x, \quad x \text { is odd, }\end{array} \quad x \in \mathrm{~N}\right.$. If for some $\displaystyle \mathrm{a} \in \mathrm{N}, f(f(f(\mathrm{a})))=21$, then $\displaystyle \lim _{x \rightarrow \mathrm{a}^{-}}\left\{\frac{|x|^3}{\mathrm{a}}-\left[\frac{x}{\mathrm{a}}\right]\right\}$, where $\displaystyle [t]$ denotes the greatest integer less than or equal to $\displaystyle t$, is equal to: