Answer:
(a) The value of  is (z+1)(3-z).
 is (z+1)(3-z).
(b) The next term in the sequence is -2.
Step-by-step explanation:
(a)
It is given that arithmetic sequence that starts with an initial index of 0. 
The initial term is 3 and the common difference is -2.


We need to find the value of  .
.

where, a is initial term and d is common difference.

The sum of an arithmetic sequence with  initial index 0 is
![s_n=\frac{n+1}{2}[2a+nd]](https://tex.z-dn.net/?f=s_n%3D%5Cfrac%7Bn%2B1%7D%7B2%7D%5B2a%2Bnd%5D)
where, a is initial term and d is common difference.
Substitute n=z, a=3 and d=-2 in the above formula.
![s_z=\frac{z+1}{2}[2(3)+z(-2)]](https://tex.z-dn.net/?f=s_z%3D%5Cfrac%7Bz%2B1%7D%7B2%7D%5B2%283%29%2Bz%28-2%29%5D)
![s_z=\frac{z+1}{2}[2(3-z)]](https://tex.z-dn.net/?f=s_z%3D%5Cfrac%7Bz%2B1%7D%7B2%7D%5B2%283-z%29%5D)

Therefore the value of  is (z+1)(3-z).
 is (z+1)(3-z).
(b)
The given arithmetic sequence is
7, 4, 1, ...
We need to find the term in the sequence.
In the given arithmetic sequence the first term is 

The common difference of the sequence is

The first term is 7 and common difference is -3.
Add common difference in last given term, i.e., 1, to find the next term of the sequence.

Therefore the next term in the sequence is -2.