Treat the x- and y-coordinates separately.
Between x=-9 and x = 7 are 16 units. Take half of that, which would be 16/2, and add that result (8) to -9. The x-coordinate of the midpoint is then x = -1.
Simillarly:
Between y = -4 and y = 8 are 12 units. Thus, add 12/2 units to y = -4, obtaining y=2 units.
The midpoint of the given line segment is (-1, -4).
Note that you could also average the given x-coordinates to achieve the same result:
xmidpoint = (-9+7)/2 = -1 (same as before).