We're looking at the difference between the coordinates here i.e. [tex]x_1-x_2 \\ y_1-y_2 [/tex]
Therefore, 2-(-12)=14 and 10-(-9)=19
Halving both of these gives 7 and 9.5, since the difference gets you all the way along the line segment, so halving it will get you to the midpoint.
Adding 7 to -12 gives -5, adding 9.5 to -9 gives 0.5 (1/2)
Putting these together, this gives the coordinates (-5,1/2)