You just need the midpoint formula, which just tells us
to average the two x-coordinates and average the two y-coordinates.

In other words, the midpoint between the two points 

 and  

is given by 

So to find the midpoint of diagonal EG, it is the midpoint between

 and  and is given by

 =  = .

As a check, also find the midpoint of diagonal FH, which is the midpoint
between  and  and is given by  =  = .

They turn out to be the same point, so  is the correct midpoint.

Edwin