Answer:
5th term is -25
and sum of first three terms is -21
Explanation:
We are given the sequence:

To find 5th term, substitute n = 5.

Therefore, fifth term is 25.
Next, to find the sum of first three terms, we will introduce sigma.

Our ak is 5-6k
Since we want to find sum of first three terms:-

Expand Sigma in.

Property of Summation

Therefore:-

Summation Formula

Thus:-
