I think the second number is supposed to be negative, -11/2 = -5.5, because otherwise the sequence is not arithmetic.
The n-th term of the sequence, starting with n = 1, is then
-6 + 0.5 (n - 1) = 0.5n - 6.5
and we want to find the number N such that
[tex]\displaystyle\sum_{n=1}^N(0.5n-6.5)=-25[/tex]
We have
[tex]\displaystyle\sum_{n=1}^N(0.5n-6.5) = 0.5\sum_{n=1}^Nn-6.5\sum_{n=1}^N1\\ = 0.5\times\frac{N(N+1)}2 - 6.5N \\= 0.25N^2-6.25N = -25[/tex]
Solve for N :
0.25 N ² - 6.25N + 25 = 0
N ² - 25N + 100 = 0
(N - 20) (N - 5) = 0
N = 20 or N = 5
