You have that A = (9,2), B = (4,-4)
In order to find the coordinates of C, a point at the middle of the segment AC, you calculate the midpoint of each couple of coordinates.
For the first coordinate:
For the second coordinate:
Hence, the coordinate of C is C = (6.5 , -1)
For the case of the points (4a,5g) and (-6a,-g), you have:
Hence, the midpoint is (-a,2g)