Approximate IIR filter with FIR filter with energy restrictions
This could be homework, so I'll only give you a few hints to help you solve the problem yourself.


Remember that a ZF equalizer just inverts the channel, so if $D(z)$ is the equalizer's transfer function, what must be the result of the product $C(z)D(z)=?$. From this equation you obtain $D(z)$, which is IIR.
Let the coefficients of $D(z)$ be $d[n]$, i.e., $$D(z)=\sum_{n=0}^{\infty}d[n]z^{-n}$$ The energy of the filter is $$E_d=\sum_{n=0}^{\infty}\big|d[n]\big|^2$$
The FIR filter you're looking for just uses the first $N$ coefficients $d[n]$. If $N$ is large enough, the approximation will be sufficiently good. According to the requirement, you need to choose a number $N$ such that $$\sum_{n=0}^{N-1}\big|d[n]\big|^2=0.8\sum_{n=0}^{\infty}\big|d[n]\big|^2$$