jmc

The midpoint of a line segment with endpoints $(x_1,y_1),(x_2,y_2)$ is $\left(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2}\right)$.

So, the midpoint of the segment is $\left(\frac{6+0}{2},\frac{12+(-6)}{2}\right)$, which simplifies to $(3,3)$. The sum of these coordinates is $\boxed{6}$.