So, we can write:
z[n] = \sum_{k=-\infty}^{\infty} c_k \delta[n-k]
Now, we are given that x[n] = a^n u[n], where 0 < a < 1. We can express the unit impulse as a linear combination of unit step functions:
\delta[n] = u[n] - u[n-1]
So, we can write z[n]
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