jmc

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

Thus, the midpoint of this line segment is $\left(\frac{2+8}{2},\frac{3+15}{2}\right)$, which simplifies to $(5,9)$. Thus, the sum of the coordinates of the midpoint is $\boxed{14}$.