(x, y)
When you are asked for the midpoint, just find the average for both the x and y components:
So the midpoint for A(-3, -3) and B(6, 6)
The xs components are -3, and 6, Average = (-3 + 6)/2 = 3/2 = 1.5
The ys components are -3, and 6, Average = (-3 + 6)/2 = 3/2 = 1.5
So the midpoint is (1.5, 1.5)
I hope this helps.