Let $f(x)= \begin{cases} x-1, & x \text{ is even},\\ 2x, & x \text{ is odd}, \end{cases}\quad x\in\mathbb N.$ If for some $a\in\mathbb N$, $f(f(f(a)))=21$, then $\displaystyle \lim_{x\to a}\Big\{\dfrac{|x|^{3}}{a}-\Big\lfloor\dfrac{x}{a}\Big\rfloor\Big\}$ is equal to: