Resposta:
D( z )
z = 0
z in (-oo:0) U (0:+oo)
4/z = 12/5 // - 12/5
4/z-(12/5) = 0
4/z-12/5 = 0
4*z^-1 = 12/5 // : 4
z^-1 = 3/5
-1 < 0
1/(z^1) = 3/5 // * z^1
1 = 3/5*z^1 // : 3/5
5/3 = z^1
z = 5/3
