Answer:
Complete proof for diagonals of ABCD bisect each other.
Explanation:
We have to complete the coordinates to complete the proof.
Mid point Formula:
Let 
 be the coordinates of the endpoints whose midpoint we have to find.
 be the coordinates of the endpoints whose midpoint we have to find.
Let (x,y) the the midpoint of given line segment.
Then, the coordinate of mid point are calculated with the help of following formula:

Given: ABCD is a parallelogram.
Proof: 
The diagonals of  ABCD  bisect each other. 
The coordinates of parallelogram ABCD are A(0, 0) , B(a, 0) , C(a+c, b) , and D(c ,b).
The coordinates of the midpoint of AC will be calculated with the help of mid point formula.
 

The coordinates of the midpoint of BD will be calculated with the help of mid point formula.

The midpoints of the diagonals have the same coordinates.
Therefore, the diagonals of  ABCD  bisect each other.