Thus, we can express the sum using sigma notation as:
\[
-3 + 5 - 7 + 9 - 11 + 13 - 15 = \sum_{n=1}^{7} (-1)^{n+1} (2n - 1)
\]
The values are:
\[
A = 7, \quad B = (-1)^{n+1} (2n - 1)
\]
The final boxed answer is:
\[
\boxed{A = 7, \, B = (-1)^{n+1} (2n - 1)}
\]