First, figure out the coordinates of each end-point.
Just read them off of the graph:
A = (9, 11)
B = (-3, -5)
Now, you have to know how to find the midpoint between two given points.
That's easy:
-- The 'x' coordinate of the midpoint is the average of the 'x' coordinates
of the two ends.
-- The 'y' coordinate of the midpoint is the average of the 'y' coordinates
of the two ends.
The 'x' coordinates of the ends are 9 and -3.
Their average is 1/2 of (9 - 3) = 1/2 of (6) = 3 .
The 'y' coordinates of the ends are 11 and -5 .
Their average is 1/2 of (11 - 5) = 1/2 of (6) = 3 .
We have calculated that the midpoint is the point (3, 3) .
I added that to your drawing.
See my attached drawing.