As per given by the question,
There are given that four point,
The point is,
A(0, 0), B(0, a), C(a, a) and D(a, 0).
Now,
Find the midpoint of the point AC.
Then,
From the formula of the midpoint,
From the point AC,
A=(0, 0), and C=(a, a)
Where,
Put the above value in midpoint formula,
So,
Hence,