6(a)
The definition of time-invariant is
If the cascade
x(t)--->[time delay by t0]----->[system]-----z(t) ---(1)
yields the same output as the reverse of (a);x(t)--->[system]--->[time delay by t0]---y(t), it is called Time invariant.
When I substitute into (1) and the reverse order of (1), the results are not the same. Thus, it is not time-invariant.
6(b)
Assuming that this is linear. X[n]=&delta[n-1]
we make the output Y[n]=u[n-1]. To get this result, the input would be X[n]=u[n].
