Geometric sequences

The third term of a geometric sequence is $-5$ and the eighth term is $-5$. $s_5$ is the sum of the first five terms and $q$ is the common ratio of this sequence. Choose the formula which is not valid for this sequence:

The sum of the first two terms of a geometric sequence is $28$ and its first term equals $2$. Which of the following statements about its common ratio $q$ is not valid?

The sum of the first $n$ terms of a geometric sequence is $-5$, the common ratio is $-2$ and the first term equals $1$. Find $n$.

The sum of the second and the third term of a geometric sequence is $6\log 3$ and the common ratio is $2$. Find the sum of the first three terms.