Step 1: The midpoint formula is:
\[
\left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right)
\]
Step 2: Using the midpoint formula for the coordinates \( A (-6, 5) \) and \( B (-2, 3) \):
\[
\left( \frac{-6 + (-2)}{2}, \frac{5 + 3}{2} \right) = \left( \frac{-8}{2}, \frac{8}{2} \right) = (-4, 4)
\]
Thus, \( a = -4 \).