In this case, the formula you would use is (a+c)/2 , (b+d)/2.
In reality, you’re using the midpoint formula. The midpoint formula is as follows:
(x1+x2)/2 , (y1+y2)/2
For example,
If you had point (2,4) and (4,8) you would plug in the equation as follows.
(2,4) = x1,y1
(4,8) = x2,y2
(2+4)/2 , (4+8)/2
6/2 , 12/2
(3,6)
So the midpoint of the two points (2,4) and (4,8) would be (3,6).